Anyone could make the hop over, age doesn't matter as they'd be in cryosleep like the folk in the main game. Not that it would make a lot of sense.
You're right.
On the other side, a lot of companions might have died during the previous three games, and saves here won't be imported (if I am not mistaken). Imagine meeting a companion which has died in the other games, people will lose their shit about it.
More interesting would be which ending was decided to be canon from the third game. The red ending (which on grand scheme of things was the only ending which made sense for the character of Shepard) means that there are no Geth in this game. If Shepard became God (the control ending), he could still be in the game.
I hope it is a clean break from the original trilogy. Sure, there should be some lose ties and some eastern eggs, but no need to return characters and so on.