It can help if you want to make a young player more determined and improve their personality. So if your senior player has 'professional' and 17 determination but your 18 year old only has 'balanced' and 10 determination it can be beneficial to try and improve it to help them reach their potential. Players can also pick up traits like 'brings ball out of defence' so it's worth making sure they play similar positions in each mentoring group.
On the flip side if your senior players have worse personalities than your wonderkids then I definitely wouldn't bother. And no need to mentor someone if they are already 'driven' or 'professional' etc.