It does work, especially if you go into the intricacies of a character that'll be a blink of an eye in the other's life, but the power dynamics there were out of control and the 18 year old definately isn't mature enough to make that kind of choice for life. In the end it worked way more with the group's relationship as a whole where everyone has a say in the journey, rather than an unnerving and overdone "oh no I can't let you get in danger, I lost too many people before".
It also makes for a very fun scene later on where the older character has to distract the younger one from certain death with his powers, and he 100% feels like he just sexually harassed her because he should have something in his incredible wisdom that could've had a better result. If they were actually in love, it'd feel really creepy, but with the new dynamic it becomes exclusively about "hey my magic gives me WAY too much power over my friends and I don't know if I'm losing control or I just couldn't make a better plan, this sucks".