You're talking to someone who you don't know very well at a cookout. He tells you that he's 36 years old. You're surprised, because he looks about 10 years younger than that. You say:
I would have guessed that you were in your mid-twenties.
age, conditionals, guess 
You're talking to your college academic advisor about which classes you should take next semester. He asks you if you know a certain professor. You think you might know who he's talking about, but you're not sure. You ask:
An oldish gentleman, built kind of solidly?
You're talking with someone who seems to be in her 30's, but she says that she has a teenage son. You can't believe that she's old enough to have a child that age. You say:
You seem awfully young to have a son in his teens.
adverb, age, children (topic), 日本語サポート You had a child when you were very young. Now your child is much older, and you are telling someone about the experience. You say:
I had Eric when I was only 18.
age, childbirth, children (topic), pregnancy, simple past 