There could be a whole host of other reasons but age really shouldn't be the case as there is a SSRI that is FDA approved for pediatric use in kids as young as 8yo. The thing about that black box warning, in my understanding, is some SSRIs were shown to increase suicidal thoughts but not suicidal gestures or suicide itself.

It is something everyone considering a SSRI should be educated on but I wonder if there might be an increased risk with people who have depression so severe that they aren't getting out of bed, performing hygiene, etc so once they start to perk up a bit they actually have the energy to consider suicide. I'm not certain just a loose theory.