Honestly I think Messi is the GOAT, but Rooney was an animal at 18. That Rooney at Euro 2004 was just insane, Messi was good at the same age but wasn't the same as Rooney that broke out at the Euros I think the best teenager at that age I've seen.
I'd say Messi 18/19 years old and onwards started to just destroy everything in his path in world football but it wasn't until Barca shipped out Deco, Ronaldinho so Messi could be center stage that he was able to shine so much. When Pep did that I think everything aligned for Messi (Messi had the ability of course but he wasn't center stage at Barca at that point).
The Getafe goal for instance for Messi when he was 19 was just insane which is for me when he started to just become the name in world football, but you have to also remember Messi at that age had a calf tear at Celtic in the CL which they had to figure out the cause. Looking historically now we know with Messi what he did from the 2007/08 season onwards its clear for all to see.
Messi had a few injuries because of his genetic disorder I believe which needed to be resolved which is why maybe Rooney pips it for me before that 2007/08 season for Messi.