Photos of Nicholas as a young adult show him with a moustache, which I see as the graduation into manhood.
I think it was essentially a question of fashion. All the Grand Dukes younger than Nicholas had moustaches, with the exception of Dimitri Pavlovich, but not beards. Group photos of Russian officers taken from 1900-17 show the older and more senior tending to have beards or side whiskers, the younger moustaches. Some facial hair was pretty much universal, however, making Dimitri Pavlovich unusual - perhaps he was one of those who couldn't grow a decent moustache!
More generally, some facial hair was the norm for men all over Europe until after WW1, the amount gradually reducing to a modest-sized moustache. I have seen it suggested that one reason for all the strange side whiskers of the 19th century was the dangers of the cut-throat razor, and that they largely disappeared with the advent of the safety razor!
I can't claim to be an expert on the subject, but the men I have consulted for research purposes (plus observations of the bum fluff among my students) for my novel tell me that it is rare to be able to grow a respectable beard or moustache until well past 20. My chap (dark-haired) says he didn't manage it until he was about 24.
Ann