Mileage can be a "relevant" thing. But knowing the history of the bike is MORE important. Age can be more relevant than mileage because after a certain number of years, parts become less and less available which means you could buy a low mileage bike that is around 10 years old and the first time you need a part, say a clutch, electronics, and other internal engine components, you may not be able to get them at all, at any price.
I will give you an example. I have been in the process of restoring a 1979 Yamaha 650 Special (XS650SF). Though the bike had only 19,000 miles on it, and even though this was one of the greatest production output bikes Yamaha ever manufactured, obtaining parts was near impossible. My best resources were private businesses that specialized in parting out this and a few other model Yamaha motorcycles that had gone out of production. I had to settle for buying used parts most of the time. Yes there are some folks that specialize in making aftermarket parts, and especially for this bike because they have had a strong following, but if it wasn't for that, I would have been wasting my time. Unless you are really good at fabrication and can make almost any part you need, you are better off buying a newer model bike that is still in production, or at least has an engine/transmission which are still in production. It may take some research, but it will be worth it in the long run. Suzuki is an example of bikes from 650cc to 1500cc that have been around for a long time. The VS700 Intruders started around 1985 or 86. Today they are 800cc engines and the name on the bike is the M50 but they are virturally the same bikes as they were in 1985-6 with a little bit bigger displacement engine. There are several other styles and manufacturers of motorcycles which fit this description. Good luck in deciding which one is for you!