I think that it is better to adopt an older child because they are mature.
They know many things, so parents can not put too much effort on the older child.
So I think an older child is better to adopt than a baby. = I think it is better to adopt an older child because they are more mature. They already know many things, so parents may not need to put as much effort into raising them. Therefore, I think adopting an older child is better than adopting a baby. 

Key corrections:

• they are mature → they are more mature for a comparison
• can not put too much effort on → may not need to put as much effort into raising them
• an older child is better to adopt → adopting an older child is better
• So → Therefore for a smoother conclusion.