For me it's about how you feel about doing maintenance or paying to have it done. I know of a couple of guys that have over 100K on their yzf600rs and take care of them really well. In those cases, high mileage just means that you'll be picking up repairs that come later in a bike's life like rings, gears, other slower wearing parts. A young bike might have those too if it's abused but you'll get more of the standard maintenance items like chain, sprocket, fluids, tires. Trick here is to get one where it's well maintained so that you don't immediately have to jump in and do all those things. There are bargains but usually because maintenance was "deferred" by the PO. Engineering factors into this as well but I don't have enough familiarity with the different makes to say which ones are good out to 60k etc.