I know your asking for our stories, but i'll give mine in a second. My advice is to find a well known eventing barn with a good reputation, and start taking lessons, go and help groom at some eventing shows, then maybe use a seasoned eventer to show until you get the ropes, then slowly introduce your horse into the eventing world.
And you are definitely NOT too old. If you're a good rider, my guess is you would start out showing at beginner novice, where in my area(GA) is mostly 11-18 year old riders with the ocasional outlaying factors.
I got started in eventing when I was 9, and was riding at a hunter/jumper barn, the owner and my trainer had a HUGE spat, and we moved barns, where everyone who moved with my trainer became eventers. We started out with my trainer(who had evented prior to doing hunters) taking us to shows, having us volunteer to help out and get everything ready with the show. Then we started taking our horses to school at eventing facilities, then we did schooling shows, then began showing at rated shows. A few years back(when I was 11 I was ranked 3rd in beginnner novice in GA) So everything we did started me out right. You will find that almost every eventer is really nice,and willing to help you out in any way possible.
And kudos to you for bringing up the 4 year old, I did that with my last mare, I just sold her to a cute little girl wanting to start eventing.
If you have any questions feel free to pm me. I'd be happy to tell you anything.
^^That was me and my pony schooling BN
^^That was me and my TB schooling training.
I can't find any more pictures. =(
Oh, and I'm sure I speak for everyone when I say we want to see pictures of this paint.