Depends on the site. Check the fine print, and I do mean read the fine print. It's not universal and each site is different. I've seen it anywhere from 18 to 21, and then even as vague as "whatever the legal age is in your province to gamble".
Which site are you looking to play at?
Yes doctor, I am sick.
Sick of those who are spineless.
Sick of those who feel self-entitled.
Sick of those who are hypocrites.
Yes doctor, an army is forming.
Yes doctor, there will be a war.
Yes doctor, there will be blood.....