I bought my guy at 21 years old 6 months ago. He's lived in my barn for about 7 years, and his owner has had him, and two other horses boarded there, for 11 years. I was lucky enough to know a lot of his history, but I still had senior blood work done, along with a dental exam, and an all over wellness exam. I just wanted to know his current state of health and if there was anything that we need to keep an eye on. At 17, I don't know how concerned I would be with the senior blood work, but talk to your vet and ask what types of things they would recommend.
I've spent most of my life riding horses. The rest I've just wasted.