According to Jonathon Van Maren, who has done extensive research on this topic, the average age kids are exposed to porn these days (via smartphone, of course) is around 9 years old.
And not "soft porn" or "your daddy's porn", but some seriously degrading, debased stuff that I won't go into details here. Let's just say they're trying to get NON-PERVERTS into sodomy, degradation, domination, violence, etc.
Don't worry, your kids are not in danger AS LONG AS THEY DON'T HANG AROUND NON-TRADS WITHOUT CONSTANT ADULT SUPERVISION.
But if they do, they're screwed. Just a word of warning.