US recruits soldiers at 17(sometimes 16 in some areas). Not Uncommon in Countries like US or others.
The US recruit them into the national defence force. State Actor.
For training and preperations. They do not enter a combatzone before turning 18. And usually wait abit longer.
It might still be "legal" to send teenagers into battle if they are part of the national defenceforce. Many countries has special rules for wartime.
Right Sector's Ukrainian Volunteer Corps (armed wing of a political party, not the national defenceforce) is just scooping up teenagers with little to none training, give them the bare minimum of preperation, and then send them into a combatzone. There is a big difference.
Even if they are only recruiting them for training and preperations it is still not ok. Non-state actors and guerillias are not allowed to recruit soldiers less than 18yo.
16-17yo soldiers and younger are usally only found in 3rd world conflicts.