Who shold have played Carl instead of Carrey?
Age differences can be overlooked as long as they are within reason. At least try to stick by the rule of thumb -half your age plus 7. Applying that rule, the youngest woman that should have been starring opposite Carrey would be in her early 30s.
Now, who would have been a better male lead than Carrey to star opposite Zooey? After some thought, I would say Justin Bartha. He is a rising star - was funny in the National Treasure series and the Hangover and is a believable love interest for Deschanel.