The apparent disconnect between class starting age and the ability to multiclass on a whim comes from the schedule of basic training for all classes. All classes are going to need to learn how to jog for several miles, carry a heavy pack, hold their breath absurdly long, and do other elementary system functions. It's just that a wizard doesn't prioritize those and makes slower progress getting to the level 1 competency stage of doing those things. Barbarians and sorcerers have precious little else to do but learn those, so they start adventuring earlier.