It may be better to think of it this way. In real life amateur racing, we have a few age categories. For example, there are separate categories for juniors and masters (usually multiple age bands for masters). Now, obviously, a lot of people in the Masters 35 category are extremely fast, but the point still stands.
Zwift elected not to go that route when setting up races. It could allow organizers to restrict entry by age bands, or something like this. At the bare minimum, it could allow Zwiftpower results to be sorted by age category, a functionality which was once present, but - and this is sort of on-brand for Zwift - which got broken in a later update. And now they are not able/willing to fix it because they’re transitioning the ZP system to something native to Zwift.com on an unspecified timeline, i.e. it will happen Zoon, if you know what I mean.
So, I agree there’s no clear way to handicap results by age. The way we do results in cycling (and lots of other endurance sports) is probably not conducive to handicapping. How we’ve handled this IRL is age categories. It would be nice if Zwift could implement something like this without breaking something else.