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 young horses are 'old souls', but you can be more assured with an older horse, that their reactions will be more predictable, and they will be safer to work around and ride or drive.
You might shy away from an older horse because you feel you will be very ambitious in your riding in the future. But, keep in mind that there is a lot of learning between you and your final goal, and an older horse may make the learning easier. An older horse can help you realize your ambitions, and get you ready for the horse that will help you carry those ambitions out. Remember too, that in a short time 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. Your older horse will be more likely to be able to adapt to a new schedule, without losing training time.
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.