Yes it mostly happens in young-er individuals because the reasons behind it are most likely genetic/hereditary and thus these people often don't live long enough to die from them later on. Some reasons include hypertrophic conditions( I'm not 100% sure on the english terms and I'm still an undergrad). Other reasons have to do with problems on ion channels which cause issues with electrical signals traveling through the heart resulting in ventricular fibrilation( e.g. Brugada, but you might have had episodes of passing out in the past). Some of this stuff is detectable but it's hard and there's no effective screening of the population. You might want to get checked by a specialist if you have a family history of early sudden deaths.

Environmental factors include things like steroid abuse( often in conjunction with amphetamin usage, see Rich Piana suspicions).