Sorry but mld81 is NOT CORRECT.
For a player to be considered home grown, he must have been playing/training in England BEFORE he is 18. If he comes when he is over 18, even if he comes on the very day he turns 18 he will not be considered home grown. This is because he isn't able to complete three FULL years before he turns 21.
mld81, you better start stocking on some u18 cuz I think you're going to have less home grown players than you think!
This homegrown rule is why it's a bit annoying when you sign any sort of South American talent. They're never considered homegrown because they can only transfer over to Europe once they're 18.