Gun powder (Black Powder) can change, but Smokeless is extremely stable. I have used a lot of pre-1946 9x19mm, 7,92x33mm and 8x57mm ammo and never had any problems.
He11, at 15 it isn't even old enough to get a Driver's License yet!!!
Yeah, as long as it's loaded, I wouldn't worry... I believe it can only deteriorate if it were exposed to the elements/atmosphere. And even then it should get less powerful, now more so. i.e. I wouldn't use it for defensive purposes... but out on the range would be fine.