The base definition of 'look' in all of these contexts are similar, but not entirely the same. 1."You look like a child acting like this"= this is used when someone acts childishly, and is thus compared to a child using like as a simile This means the person isn't a child, but is compared to one due to his behavior 2".You look very handsome"= describing the person's appearance and used literally. It means that the person saying it to someone else genuinely thinks that (except if he says it sarcastically) 3. "You still look good for your age"= once again, simply describing outer appearance

We NEVER say "you looks", ever.