I changed apartments 2 1/2 years ago, and subletted my first apartment to someone for the remainder of the lease.
Now I received a letter from a collection agency stating that I owe rent from 2 1/2 years ago, so I guess the subletter failed to pay rent and it appears I am held responsible for the amount owing.
I live in Ontario. Does the statute of limitations of 2 years apply in the case of rent debt?

First, the SOL would go from the when the sub tenant stopped paying, not when he moved in. But, yes I agree that this would fall under Ontario's 2 year SOL.

However, that doesn't mean they can't still try to collect. An SOL simply means they can't file suit against you. The debt still remains.

So you need to find out WHEN the breach occurred. If it occurred within 2 years, they can still sue.