What is the age limit for competing at an Equestrian Olympic event?

As far as I am aware there is no age limit for competing at an equestrian Olympic event nor should there be.  As long as horse and rider are both fit.  In fact Mary King, who I believe is 50 has said she doesn't feel like retiring yet and may even consider the Olympics in 2016. Mary King has an impressive record with 5 Olympics under her belt from 1992 to 2008.

Mark Todd actually retired from eventing in 2000 but has since returned in 2008 at the age of 56 to qualify for the Beijing Olympics.  He also won gold medals at Los Angeles in 1984 and Seoul Olympics in 1988. He has also won Badminton on four occasions, the Burghley Horse Trials five times and as a member of New Zealand 's eventing team he won gold medals in the World Championships in 1990 and in Rome in 1998, the European Championships in 1997 plus 20 or more other international events.

On 25th April 20011, Mark Todd competed a fourth Badminton victory riding NZB Land Vision becoming the oldest winner of the event.

As a lesser talented rider at the age of 54 I have begun to think about retiring from competing but once I get on the cross country course this idea is quickly forgotten although going round last weekend on a very green horse was a challenge. I hasten to add the horse recovered in two seconds and I was still blowing ten minutes later!

My grandmother in fact was still riding but not competing well into her 70's.  After having tripped whilst on foot to open a gate and breaking her ankle which she recovered from and returned to riding she then fell off and had concussion.  It was at this point she decided to retire as the hospital were becoming quite use to ringing me to say 'your grandmother has fallen off again'.  She did carry on having a small pony as a pet until it became too much for her to look after, but she was a game old bird.

I can also remember for my mother's 70th  birthday taking her for what was going to be a gentle hack.  She had not been on a horse for some years.  Once on board we started very gently. I could see she was itching to go a bit faster once we were off the road and I asked her if she wanted a canter.  Her face lit up as she said 'shall we'.  So off we went up this hill.  I could see she was really enjoying it and certainly hadn't forgotten how to ride.

Once back at the yard she did have to be helped down as her legs were a little wobbly, but at least she did it.
I also know of one old lady who was still riding to hounds in her 90's she only gave up because she couldn't get the ramp up on her trailer on her own!