How old is he???

I love my 20 month old son very much, the issue we have with him though is that he doesn't understand that he is 20 months old....he thinks he's 4 or 7 like his siblings. He has tried scootering like them, tried getting on Josiah's 2-wheeled bike, he swings like them, goes up and down the playground set like they do, thinks he is running as fast as them, sits at the counter on our stools because they do, wants nothing more then to ride in the van in a booster rather than his car seat....he doesn't understand that there is a difference between his age and the ages of Josiah and Alayna....if only this could work in my favor when it comes to using the bathroom....