It's much like pointing out that a child does not ethnically resemble his parent.

The first time you tell someone that they look too young to have had whatever child they have, it's a compliment... or, at least, can be meant as one.
The second time you make the comment, it becomes mildly uncomfortable to the target. Target is the appropriate word here because the statement has become pointed.
The third and every following occurrence, you are simply begging for an awkward conversation about teen pregnancy and what it feels like to be a statistic.
Are you looking for some sort of confession?
Are you my priest?
Did my Catholicism unlapse without my noticing?
Have some couth.