The thing is that we go from "we don't check the age at all" to "children now need to work around an age verification" system. Seems like it will be harder for children, which is the goal.

Then make it illegal to sell them. Some people will still do it, but children can already order cannabis over the internet.

It's always a trade-off, it will never be perfect. But the status quo is not perfect either. The question is: is it better than the status quo? I think that age verification is not completely unreasonable (as long as it is made in a privacy-preserving manner). As a comparison, I think that ChatControl is completely unreasonable.