The date of Thorondir's birth appears only in The History of Middle-earth volume XII, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
The thoron in Thorondir's name is surely 'eagle', but -dir is open to interpretation. It is often simply a masculine name ending, so his name may mean no more than 'Eagle-man'. On the other hand, it can also be used in an 'agental' sense, suggesting that Thorondir actively used eagles in some way, presumably as hunting birds.