Five years is a long time, and your interests can change. The horse you bought because you wanted a trail horse is no longer suitable because now you want to learn dressage. Or in five years your life might change dramatically and you no longer have time to ride four nights a week and can only ride a few times a month. Buy a horse that you can get on and enjoy right now, even if it is an older horse.
Young horses are often less steady, and have less training and experience. Their reactions are not as predictable as an older 'been there, done that' horse. Young horses need riders or drivers who can foresee problems before they begin. Beginning riders rarely have the foresight or knowledge it requires to deal with a young horse safely.
Some older horses may have physical conditions that require extra care. But as long as the horse can be maintained so it is comfortable and the treatment isn't draining your bank account you may decide the extra effort is worth it. The pre-purchase exam should reveal what is entailed in maintaining any horse with any unsoundness.