When my nephew's car broke down and I loaned him my car, I know that he'll be careful with it.
Well at least you can trust him to be careful.  I dont think I would be able to trust my nephew as he is only 18 and not very responsible yet.
Yeah, 18 is an inherently irresponsible time, mine is 24 and getting married soon so that makes a lot of difference.
Yes its amazing how much of  a difference a few years can make in age.  It seems that males take quite awile long to mature than females.   
The culture that extends adolescence deep into their twenties doesn't help much either. 