I'm sixteen, and I recently made the switch.
One thing for sure--in western riding you do use a lot of leg cues. The problem is the saddle makes it harder for you to do so. Therefore, most cues are brought on by reining.
I think horses and people should be double-sided. I know western pleasure horses that direct rein. The old horse I rode had to be neck reined, otherwise he would wish to go quickly. Switching helps define your seat, and your balance especially, and it can be a little scary if you're on a big horse to have nothing supporting you but your own seat.
It took me about a day to 'learn to ride'. The biggest thing I have noticed is your ability to be closer to the horse, and therefore move with the rhythm. Posting may seem a bit odd for you at first, but concentrate on going with the rhythm in all gaits. One difference is the walk especially, and to keep an upright walk that is engaged, rather than the mosey-ing along a trail sort of walk pleasure horses may have.
I hope I'm making sense.
But let me make you aware of one thing. Riding horses is like figure skating, dancing, and many other hobbies that are often begun at a young age. But just because a lot of people start when they are two or three does not mean that everyone does, nor does it mean that you do not have the potential to catch up to their skill level in a shorter amount of time than it took them to learn. As a young adult you will be more educated in horses in general and have a good foundation, and probably catch on quicker, than a little kid would. Don't worry at all about your age. I know plenty of adults that begin when they are older.
As for questions, make sure you find a trainer you like. Please. From the way you sound, it sounds like you have been in a family-filled pleasurable environment. Some trainers can be hard and expect the best out of you, and be like a military official. For some people that is good for them and gives them a challenge in confidence. However, some trainers can be like your best friend, and help you by gentleness and sweet words. Don't worry so much about what they are teaching you at first as to how you like them. If you feel stressed out at any time, or uncomfortable, find another trainer.