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%.