Children’s brains develop very quickly so the change is size is quite fast. Human brains seem to be as big as they get around 21 years old, but between 2 years-old and 21 years-old the brain size only increases by approximately 20 percent. Although the brain doesn’t get noticeably bigger after that, the neurons (brain cells) that the brain is made-up of are always forming new connections and loosing old ones. It is really more exciting to look at how the function of the brain changes as you get older rather than its size. You can lose quite large bits of your brain and do the same things as other people, so size really isn’t everything when it comes to the brain.
I think Tim’s answered this really well. Actually, percentage wise, a babies brain is bigger than an adults, but as you know, adults can do a lot more than babies can! So Tim’s right, size doesn’t matter!