Statistically, sociologists use/d specific markers to determine people reaching adulthood. Interesting - and unsurprisingly - people become "adults" at an increasingly old age. Because people marry later and later, move out from their parents later and later, have children later and later and so on.
This trend will probably just continue. In that sense it's also hard to tell when someone becomes an oldcel. My father first married with 17 and then again with, err, I think 25. (My mother). Then he worked, then they built a house.
For him, a 30 year old might have been old ... but these days, it would feel kind weird and wrong to say that a 30 year old incel is old.
So I guess ... maybe 40?