I agree with Trav about it being an individual thing. DD1 got a bike at 3.5. She was chomping at the bit and could pedal easily. We got her a 16", but she is a seriously tall kid and we've already had to upgrade to a 20".
DS has shown no interest in a bike. Well actually we tried to get him on one in the shop for his 3rd Birthday and he ran away lol. He's only just learned to peddle and shows an interest now at 3.5....but not enough to want to 'try' a bike. Santa's bringing him one.

I don't think your DS2 will even notice the difference TBH. Christmas is usually such an exciting day, it's usually us that worry more about the actual pressies.