During one of the episodes in season one of Deep Space Nine, Sisko asks O'Brien to find his son. Since Jake doesn't have a combadge, Chief must've found him by recognizing he's the only young human on the station. That would be the best way to locate him.
We know from the original series episode "This Side of Paradise," that Starfleet scanning technology can identify substructures in human bones. (Doctor McCoy checks the soundness of his equipment by running a check on himself, and it picks up that he had previously broken his ribs.) Consequently, it should be possible to identify an individual as not an adult, but noticing that the growth plates on their limb bones had not closed. That is a very crude of measuring age (distinguishing adults from pre-adults), but it is clearly within the abilities of Starfleet technology.
Medical scanners can do so. In "Rascals" (the one where Picard and co. are turned into kids), Dr. Crusher determines the effective age of the team to within a few years, and is even able to isolate the specific de-aging technobabble in play.
On the other hand, in "Collective" (the one with the Borg kids) Voyager's sensors are apparently unable to tell that the aforementioned Borg kids are, in fact, kids. (They detect the Borg neatly enough, and can isolate the right number, five, so it's not jamming or sensor failure.) This doesn't necessarily mean they couldn't distinguish between children or adults in normal species, though. It might also be that the sensors or sensor operator could distinguish them if they had reason to look for children; Voyager had no idea Borg kids were even a possibility until they investigated.