I once read a very useful advice on this. Monetize from the beginning, because you want subscribers who are alright with seeing ads when watching your content. If you monetize later and your subs are not used to it, you will lose the impatient ones. And you dont want these in the first place (not to sound mean ofc, but most youtubers want some profit from youtube, so yeah, subs who dont want ads are not the subs youtubers who want profit want as subs).
Also, in case you get a viral video, you're not getting money you could have otherwise have gotten.