Tires dry out fast here in the desert... I try to use them up in under 4 years.
Other places may go 6 or so...
A little surface, heat checking on the sidewalls ain't so bad,.. 'till it starts to get deep...
...keep in mind,... the problem here, is that a tire has petroleum in it... that's what evaporates out... and when it's gone, tires don't stick anymore...
...(that's also what moves around, and creates flat spots, if your bike sits for too long without rotating)...
If you dig a thumbnail into a corner edge of tread in a good tire,...it will smoosh a bit, and leave a slight nail imprint... (like a new pencil eraser)...
A dried out, dead tire, will feel hard,...more like Tupperware, or an old pencil eraser that don't work anymore...
A tire like that, has no traction at all,.. and is dangerous...
Tires start drying out, from the day they are made...
... even on the shelves at the store...(they're not supposed to sell tires stocked more than 2 years)...
(check date codes on new tires)...
A 1 yr old tire worn down to a slick, is 10 times better than a 10 year old tire with full tread...
Ditch them old tires...
...
Tread ain't there to apply traction,... the meat of the tire has all the traction,... tread is just there to move aside rain, sand, or mud, to get the tire down to ground surface... a slick has the most traction...
If you ride in deep sand or rain a lot,... you need to watch your tread depth more than us, "fair weather" riders...