Wouldn't bother me too much. 17 is relatively young.
Keeping them fit makes a huge difference. Just like active people. Good diet, all around maintenance care, farrier work, genetics, etc. all play a role in life span.
My old man is 29, 100% sound & healthy. He's still ridden several times a week and has even moved & worked some cattle recently. Only thing different for him than my others is he needs more frequent dental work & chopped hay.
I also have 2 mares in their 20s at 24 & 23, both very easy keepers. One has very mild arthritis and is easily managed on MSM & Desert Yucca.
I love my seniors, they are a great change of pace from all the youngsters I ride.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.