How do you know when to replace a tire if it still has some tread left? How can you tell if it is too old ?
Replace if you are not comfortable with loosing traction. There are wear bars on M/C tires too just as on C/Ts.
Are there cracks in the sidewalls or in the tread.
If you are going to ride that tire out a bit more, check it very often for pressure, cracks in tread, cuts and punctures. Do it each ride when the tread wears thin and uneven like that. Put the bike on the centerstand and spin both tires inspecting them.
Age.... after 5 years, Trash them especially if riding any distance at speed.
At the end of the tire vin is a raised box with 4 numbers in it,
first two are week of the year, 01-52
Second two are year of MFG
I'm running a Michelin Active (Rear 130/70-18) backwards on the front, has over 12,000 miles on it and still lots of tread and very minimum starting of cupping, it handles great now that it is broken in. It has deeper tread than the front tires.