tbf mbappe was already a champion with monaco at 18 and scored in a CL semi-final. he was a WC winner at 19(and an important player in that win). also played a CL final already.
messi and ronaldo weren't at the same level at 18/19 years old.
CR7 made it to Euro finals and Messi Copa finals at the age of 19 (or barely 20), both had inferior teams to that France team in 2018.
Messi won CL at the age of 18 too, was key player before injury, a starter in team that win CL and Liga before getting injured.
CR7 had breakout year at the age of 21, taking his game into a new level that Mpabbe never did so far.
That said, it is unfair comparison, Messi is arguably the most talented player ever, while CR7 is probably the most athletically gifted too. Both started at the top (since 18 years of age, one was at Manu, other in Barca) and had great coaches and teams around them. The stars was aligned for those 2 to be historically dominant like no other