Young iguana has begun to trill

We received a young green iguana back in July that we were told was about 9 months old.

I'm starting to wonder if Hannibal is older than what I was told. He has a nice layer of spikes down his back and just looks like a grown iguana in a small body.

This last week, when I'm holding him and skritching around his face and head, he's started to 'trill."
Now, I have had grown iguanas make this soft purring sound, but I've never heard it from a young one.

About how old are males when they begin to trill??

Anybody? (I know. I sound like a total noob)

Just not sure if I should be flattered or concerned, ya know?
The bigger ones we fostered made the sound when they were content, so I guess this is the one time Hannibal is actually happy. (He's usually a pretty evil little guy)

I guess I'm just trying to get a better idea of about how old he is.