IMO - I would enable ads from the start. I asked a similar question on Reddit and most people said along the lines of: "yes as there's not really a reason not to." If you later enable ads when your channel kicks off then people will think that you're a "sell out" even though you would only be doing what everyone else would.
Also now, with the new format, you need 10,000 views or more to start showing ads on your videos (as far as I am aware) so that's something else to take into consideration.
But yes, I would enable them as early as possible.
Hope that helps.