It’s a grey area. The regulations don’t mention tire age at all, or even dry cracking, unless it’s so deep that there are cords visible.
Saying that, I had a discussion with a government MTO officer that was doing a spot check some years ago about this very question. According to him shops are within their rights to fail if their personal judgement deems the tires unsafe.
As for the official verbiage, I copied the motorcycle tire portion of the regs below (
Law Document English View)
(3) Each tire shall be inspected for depth of tread, tread and sidewall defects, regrooving, proper size and application, and,
(a) no tire shall be worn to the extent that in any major groove at three equally spaced intervals around the circumference of the tire,
(i) the tread wear indicators contact the road, or
(ii) less than 1.5 millimetres of tread depth remains;
(b) no tire shall have exposed cord;
(c) no tire shall have tread or sidewall cuts or snags deep enough to expose the cords;
(d) no tire shall have any abnormal visible bump, bulge or knot;
(e) no tire shall have been regrooved or recut below the original new tire groove depth;
(f) no tire shall be of a smaller size than the motorcycle manufacturer’s specified minimum size or be sufficiently oversized as to contact any vehicle component so as to affect the safe operation of the vehicle; and
(g) no vehicle shall be fitted with a tire that,
(i) bears the wording “not for highway use”, “farm use only”, “competition circuit use only” or any other wording or lettering indicating that the tire was not designed for highway use, or
(ii) bears the letters “SL”, “NHS” or “TG” after the tire designation.