As long as you recognize the limiatations of your individual horse there is no reason why not. Mine just turned 17 and we've brought it down to 3'0 from 3'6 since he's begun to knock poked and show more stiffness after work. Every horse has different! There are plenty of maintenance options that can keep an older horse comfortable, just know that older horses may need more time and attention (longer warm ups and cool downs, etc) to keep them kickin
Id ask your vet to look at her legs and joints. My 28 y/o pony was jumping 2' without a problem before he was diagnosed with cataracts. Age doesnt necessarily mean they have to stop, it just means you have to be more careful.
I would get a vet to do workups (flex tests, bloodwork, etc.) and just be careful and keep up with conditioning work if you get the all-clear, but she's not that old :) Flexible's still jumping after all!