They don't need new recruits as badly as they need arms for the soldiers they already have. What are they gonna do? Send new arrivals to the front with slingshots and have them shout "pang! pang!"?
Ukraine's conscription starts at 25, despite the USA throwing a fit about them not conscripting younger men, by the way. The citizenry refused to budge. That is what I mean. Ukrainians do have some say in government. It is _not_ Russia.