A large number of men will grow into their mid 20's so it is very realistic.
The only growth that men do after they turn 20 is outwards, not upwards. haha.
in all seriousness, in my final year of high school, which had 200 students aged 17, only one guy grew massively, but pretty much everyone's height was set by the time they finished.
the % of men that grow in height ... at all... after 17, is probably less than 10%.
10% is probably a high number too. I'd go so far to say 5% would be me going high to be safe. I only know of 1 person total who grew after age 18. He grew 2 inches between 20 and 21, despite 18-20 staying the same height.
I know more people who have been 6 ft,- 6'4 range since they were 14 years old, never growing after 14. Than people who grew after 18.