A 14 year old needs to get at least 9 hours of sleep but some 14 year olds might need 11 to 12 hours of sleep.
As long as the 14 year old is getting 9 hours of sleep they should be just fine and be able to function properly the next day.
If the 14 year old needs to be up by 7:00 AM in the morning then ideally they should be in bed by 10:00 PM at the latest to allow enough time to sleep to get 9 hours of sleep.
It's best to get into bed 15 minutes before 10:00 PM to wind down and allow enough time to fall asleep because it can sometimes take 15 minutes to actually fall asleep once you get into bed.