The ex-central defender and Arsenal ace, 44, left a fire emoji on one of the girl’s posts last month.
In the photo, the model poses on top of a bed wearing a black bra and ripped-up jeans.
The agency she works for confirmed to Daily Star Online: “She’s under 18 with a public profile.”
There is no suggestion that Sol, who now manages Macclesfield Town FC, would have known the girl’s age before leaving the emoji.
The married dad-of-three later deleted his comment but not before dozens of other users had noticed.
An Instagram user wrote: “Who else is here to try and find Sol Campbell’s comment?”
Another said: “I’m here for the Sol Campbell comment.”
One asked: “Where’s Sol Campbell? I want to see?”
“Who else is here to try and find Sol Campbell’s comment?”
Campbell’s agent, his PA, and Macclesfield Town did not comment when asked for clarification on the emoji.
His wife Fiona Barratt said: “We are not interested in you writing a story about Sol or anything but thank you very much for your call.”
Campbell was named Macclesfield manager in November last year.
He was previously vocal about his desire to move into management and told his first press conference: “I will bring all of my wealth of experience [to this job].
“I know football inside out. I know how to manage people. I can communicate and I’ve looked at what I can do at the club.
“I think it’s a great place for me to start. I know some Macclesfield fans will probably be saying ‘what is going on here?’
“But you’ve got an international footballer, who has been one of the best footballers in the world, coming to your club. I think it’s a nice mutual balance there.”