An honest personal assessment: the bird has flown on your days as a pure coder. Even if you don't particularly want a management position, focus on that aspect of your career. It's going to be hard to convince a prospective company that you're worth the money as a developer if all you have to offer is 12 years of ASP.
I ran the software dept for a small consulting company and yes you'd probably never get a call back from me if you were applying purely as a developer. While we have hired late 30-somethings developers, it's usually because they offer some fairly unique tech skillset (i.e., a few years of production-quality FPGA experience, not a couple months dorking around with "whatever hot new technology" on github--that's something I look for in a fresh-grad). So I think for a purely tech role, beyond getting another job at a similar company doing similar tech, you're a long shot at best.
But beyond that, the one thing that would case me to look at your resume is if you were wanting a leadership position and had the corresponding soft skills and experience. It sounds like you do, so highlight that, and work on it. The advantage there is that management skills: talking to stakeholders, setting expectations and timeframes, finalizing deliverables...are all applicable no matter what the underlying technology. So there's no reason you'd have to limit yourself to ASP management roles.
My previous company had lots of young developers doing engineering with lots of new technologies, but they'd have never delivered a thing had they not been guided by some experienced devs who actually spent most of their time in Excel (or trello or Jira or whatever we happened to be using to manage the particular project).
You mention age, but then go on to say skillset. Do you mean to say regardless of skillset you are going to pass on 30-somethings? Now that the computing field is finally aging, there are going to be more and more 30/40/50 somethings pure coders. Passing on them because age alone is a huge mistake in my mind.
This is distinctly the opposite of what I said. We have hired 30-somethings based on skillsets. We have also hired 30-somethings based on soft skills. We would not hire 30-something "pure developers" based on "potential, potentially outside their skillset" though because it's far cheaper to hire fresh grads, and you just have to assume that someone in their late 30's that's looking to change is just a burnout. It's a tough world.
I have programmed on all kinds of stacks, and while I understand a lot of the .NET disdain, I’m perplexed by perceptions that years of .NET experience is somehow worthless or less than noteworthy? Beyond the stigma of being a Microsoft product, there seems to be a misconception that .NET devs are dragging and dropping junk through an IDE. Why wouldn’t their "12 years of ASP" include obtaining good debugging habits, writing readable code, building production ready systems that still perform at scale, and a knack for constant improvement? Someone who is a craftsman on one tool will share many qualities with a craftsman from another.
I don't think it's a disdain for .Net. It's more of a disdain for ASP.Net Webforms. I'm definitely not a youngster (I'm 41). But he's been doing Asp.Net Webforms for 12 years. He hasn't shown a "knack for constant improvement". He hasn't been keeping up with the frameworks and technologies that MS has been pushing - Asp.Net MVC, WebApi, Entity Framework, etc.
If he has been doing WebForms instead of MVC, he's probably not up on all of the basic table stakes client side stuff like BootStrap and JQuery.
I know at 41 years old, if I want to stay in development and not go into management and command the salary I want, I can't be complacent. The minute that my company stagnates, I must find another job. That means for me, being a full stack .Net developer:
1. Web - Angular, JQuery, CSS, TypeScript, and Bootstrap
2. Server side web - Asp.Net MVC, WebApi, WCF
3. Knowing how to speak the language of an architect (DDD, Design Patterns,everything that Martin Fowler writes)
4. Database theory and maintenance and EF.
5. Testing - front end and back end automation testing.
I'm not bragging, I know lots of developers who can tick off these checkboxes. If you're not willing to aggressive learn, this isn't the field for you.
I just wanted to add that I do have professional WebApi, Angular and JQuery work on my resume, but the big thing that seems to be the blocking point is MVC. I don't code traditional webforms development anymore (I don't use controls anymore, it's more JS calling json services), but there's no resume-friendly name for it because I'm not using a specific framework, and I'm guessing that to an HR person, all that means is "not MVC".
It's a crap-shoot on which technology is going to take off. Microsoft was pushing heavily that MVC was just another way to do things during the events I went to. I tried to push the boundaries of what my company let me work in, and that just happened to not be where the market went.
In hindsight, keeping my eye open on the job market is something I should have done more.
Another honest assessment: if MVC is the only thing blocking you from various better positions, just say you've got it on your resume. Then spend a couple days after work going through MVC tutorials. "New Project->Web->MVC->OK. F5." Bam, you've got MVC experience.
Seriously it's way easier than webforms so if you've got that much webforms experience, you'll have absolutely no problem picking it up. Within a week you'll know 95% of what you'd need to know on most tech interviews and you'll do fine at your new job. Stuff varies so much company-to-company that MVC will not even be one of the top five differences.
In my previous response I was thinking more you were looking for some big change into something very cutting-edge. That would be a more difficult proposition. But if you're just looking to go from webforms to MVC, or even to a Java-based infrastructure, it's not a big hurdle. Like others have said, the HR filter will be the biggest problem. There will be an age bias too, you'll just have to deal with that.
FWIW I don't think you should expect a paycut or a cut in "rank" either just because of changing technologies. You'll get caught up on the technical side very quickly. Outside of extremely technical companies, "rank" is far more about knowing how to get through a release cycle coordinate teams than it is about specific technical knowledge.
All that said, just be warned, working on websites in MVC (or really any technology) is really no more interesting than Webforms. (In fact webforms may be more interesting because you get to invent your own way around its inadequacies).
I'm not saying that the world is fair, or unfair, or even that I'm fair, or unfair. I'm just saying that as a person (with limited time as we all are) who works for a small fairly innovative consulting company, a resume that shows 12 years of ASP dev will get pushed to the back of the list. I don't have an HR dept. That person may be a perfect fit. This is the guy's best possible chance. Nonetheless, I have limited time and will interview fresh-outs or jr devs with more diverse skillsets first, and an old-dog ASP dev only as a last resort. Diamond in the rough? Well, I have no idea about the diamond thing, all I know upfront is it's certainly the rough, and probably not worth my time.
I ran the software dept for a small consulting company and yes you'd probably never get a call back from me if you were applying purely as a developer. While we have hired late 30-somethings developers, it's usually because they offer some fairly unique tech skillset (i.e., a few years of production-quality FPGA experience, not a couple months dorking around with "whatever hot new technology" on github--that's something I look for in a fresh-grad). So I think for a purely tech role, beyond getting another job at a similar company doing similar tech, you're a long shot at best.
But beyond that, the one thing that would case me to look at your resume is if you were wanting a leadership position and had the corresponding soft skills and experience. It sounds like you do, so highlight that, and work on it. The advantage there is that management skills: talking to stakeholders, setting expectations and timeframes, finalizing deliverables...are all applicable no matter what the underlying technology. So there's no reason you'd have to limit yourself to ASP management roles.
My previous company had lots of young developers doing engineering with lots of new technologies, but they'd have never delivered a thing had they not been guided by some experienced devs who actually spent most of their time in Excel (or trello or Jira or whatever we happened to be using to manage the particular project).