Subaru dealer website says

"Free lifetime loaners guaranteed - We have a large supply of new cars available to all our service and maintenance customers. You must be 23 years of age or older."

That says you have to 23 or older to take a loaner vehicle when your car is in the shop.

Check the leasing terms to see the other details.

Why 23?

Because they have determined that the older you get, the more responsible you are. That's why your insurance goes down as you get older, and married, and why most rental places won't rent cars to younger people (Though I remember some age-discrimination lawsuit about that years ago, and the rental companies lost).

It's so they're not loaning their brand spanking new Impreza out to the 16y/o who is in getting his car fixed from his first accident. They don't want his second accident to be in their car.

Why 23?

Because people younger than 23 aren't old enough to understand.