Is it too late to make a claim if I closed the deal months ago?

Not necessarily, but this needs checking against two separate clocks rather than one. First, an indemnity claim under the purchase agreement is generally bound by the survival period the parties negotiated for that specific representation — some run only a year or two, while fraud or fundamental representations are sometimes given a longer or open-ended survival period, depending entirely on what was agreed at the time. Second, Ontario's general limitation law runs on its own separate discoverability principle, generally starting when a claim was discovered or reasonably ought to have been — which doesn't automatically match whatever survival period sits in your purchase agreement.

A claim can be alive under one clock and already closed under the other, so both need to be checked specifically against your agreement and the facts of when the problem actually surfaced. A few months after closing is often still within a reasonable window either way, but that's not guaranteed for every representation. Don't assume time has run out, and don't assume it hasn't — get the actual dates checked promptly.