Age really shouldn't be a factor if a person has dedicated themselves to a martial art (whatever style). Look at the founders of Aikido, Jujutsu, Judo and EPAK for starters (Parker wasn't as old but he was in his late 50's and going strong before he died... of a heart attack). Then look at Yip-man and his son Yip-sing, both were still practicing and teaching their art (Wing Chun) at an advanced age. I had the privalege to go hands on with Yip-sing on learning a particular techinque and I'll tell ya right now, at 80+ years old that is one guy I would not want to seriously mess with; there was nothing wrong with that man's speed or strength.
How well you keep yourself in shape and how well you avoid practices that'll bring on arthritis and other age wearing stuff (whatever!) will determine how long you can study that particular art. If you're young and go for a hard style, then stop for about 10-15 years then want to get back into MA... then it'll depend upon how you lived during that hiatus. Did nothing but a desk job 9-5 every day and the most strenuous activity on the weekend was mowing the yard with no other exercise ... then yeah you'd probably, MAYBE want to go to a softer art. But if you were the opposite or had a hard-physical job and went say Rock Climbing every other weekend and exercised reguarly then no reason is there for you NOT to get back into whatever hard-art you were doing before or a newer one.
You make the determination on how "old" or how weak you'll become in your advanced years. The saying is very true, you're as old/young as you feel. Taking care of yourself and your body throughout your life is tantamount to the rest of it (marriage/family/career and so on). I've known guys 70 years old that beat a bunch of 20-30 somethings on a 4 mile-1500 foot elevation gain hike. Simply because he took care of himself.
Again, it's not the art but the artist.