It will take some dedication and work, but it's not at all impossible. Here's the obituary of Walter Gervais, who started eventing way later than 47! WALTER GERVAIS | The Chronicle of the Horse
Seriously, though. At a show about 10 years ago in the warm up for a top division (maybe advanced, but it could have been a CIC***), a rider with limited miles at the level saw Bruce Davidson--who evented in his first Olympics in 1972, was world champ in '74 and '78, and had been at that level on myriad horses since, take a deep breath before XC. The rider turned and said something to the effect of "you still get nervous?" Bruce replied that the day he was not a bit nervous coming out of the start box was the day he was too foolhardy and needed to quit eventing.
We ALL get butterflies, but it is precisely those butterflies that make completing XC so much FUN! Throwing your heart over the fence so you have no choice but to ride over after it, the satisfaction of a course well ridden, the pride in your horse when he saves your sorry butt, the pride in yourself when you convince him that yes, that really is the jump you want, the feeling of having your horse locked onto a fence or, better yet, looking for the next fence to lock onto . . . . those are the reasons we event.
Oh, yeah, and we do some prancing and stuff first and we get to jump again. That part is okay, too.