Hmm, I'm 45 and it doesn't seem hard. I think adults have a lot less time and more stress rather than some hobbled brain.
Children do it naturally, given a right environment, and simultaneously for two or three languages.
That doesn't mean it's impossible. "Hard" is relative. But, children's brains quite clearly have abilities that are lost later on.
"Research comparing children to adults has consistently demonstrated that adolescents and adults perform better than young children under controlled conditions (e.g., Snow & Hoefnagel-Hoehle, 1978)."