In terms of 'having a decent life and not being fucked up mentally'...I'd say if you don't get laid by age 20, it's over. Even if you do get laid after that point you'll forever be bitter at what you missed out on. Teenagers here take note.
In terms of not being able to fuck prime women (i.e. up to age 21), you're taking a hit on your chances once you're age 25, and it falls off a cliff once you're 30. If you happen to look younger than your age though, while this was a death sentence at college, you might be able to stretch out an extra few years here of your 'prime' by agefrauding. So my boundaries of 25/30 assume you actually look that age.
In terms of being able to fuck any attractive women at all (i.e. up to late 20s, as women hit the wall once 30+) then I'd say age 40 for a guy is the point of no return. Again, this assumes you actually look 40.
After 40 you're basically just restricted to hooker/golddigger game if you want to fuck prime women, which anyone can play irrespective of their looks.