That page says discrimination during recruiting "may" be illegal, and that you "shouldn't" discriminate on age. It should say "is" and "mustn't" if it's certainly against the law.

My main problem with gov.uk is this fault to make clear the position of its advice in respect of the law, IMO it should link the appropriate legislation/caselaw as citations to prove the position it contends.

It is okay to discriminate for some jobs if the position requires it e.g. you can't be a bar tender if you're < 18.

You can't work full-time under 16yo, so all jobs require age verification??