A few weeks ago our older Dam Topeka went into heat. Her last litter was a bit more difficult, so that as well as her age made us decide to spay her.
We waited until she had another heat cycle, and then spayed her.
Some things to keep in mind when spaying an older Dam - especially a Boxer:
1. Boxers are very sensitive to an anesthetic called Acepromazine.
2. Older Dams have a harder time going under anesthetics. Talk to you vet about your options.
We chose to use gas instead of the anesthetic. She did extremely well and had her appetite back the following morning.