So the 'normal' thing for applicants is that they have to declare children and have them examined when they apply for and become PRs. And if they're not declared, they cannot be sponsored.I got my PR through ICC. I didn't know that I had illigitimate child can I sponsor him now? Thank you for the advice.
Situations like yours are rare so it's hard for anyone to say precisely what the situation will be. Given that one would have an incentive to say 'I did not know of this child', whatever happens there will be some extra scrutiny. (If you're not listed as the father on the birth certificate, an extra complication too - and they may ask for DNA evidence regardless).
Now: I think it should be possible to sponsor the child - but it may not be easy. But I'd strongly suggest getting a consultation with a lawyer about how to proceed. You may need to provide evidence and legal docs about this (affidavits you didn't know, how you found out, etc). You don't want to make an error that will close the door on this and should get professional advice - IMO.
And note: I haven't even touched the issue of custody and if you're prepared to take on full custody, permission of the other parent (who cannot be sponsored), etc. You'll also need to show that you can support one more child (in an already not small family).
One caution: while I'm sure there have been other cases, this type of situation is outside the normal bounds and some specifics may matter a lot - in other words, cases that seem similar may be quite different from immigration regs and law perspective, so don't jump to conclusions that it will all be easy if you hear it worked for someone else.
This is why we were asking things about how you became a PR/citizen etc - specific case circumstances matter. It's not all bad - the fact that you were fairly young at the time could be seen as a positive (although you were still an adult, it's not uncommon); the country/culture where this happened may matter (eg family of the mother not wanting to disclose/publicize what happened); and the fact that 13 years or so has passed since the child was born (and that you started another family) may support that you truly did not know about the child.