Food Sharing - when do they usually start?

[Summer1216]

I'm curious about something. I can't find any information online about when babies start trying to share their food back with others.

My DS is 9 months old. This morning I was giving him some of my sweet potato with bread, and before he'd eat it, he wanted to hold it up to my mouth. I made a lot of slurping and smacking sounds which he found amusing. 

I remember my nieces and nephews going through a phase where they wanted to give me parts of slimy cookies and drooled on apples. Do you think DS has hit that phase, or is he too young for that??? 

If he has hit this phase, then my next question is - my two closest friends have kids 14m and 18mo, and neither of their babies try to share food. I'm curious about why - because I thought that 1y-2y was when they usually shared food the most (I remember a lot of slimy teething crackers, which I thought were kinda gross at the time). I know kids go through different developmental phases at different times, but I would have thought with them be so much older that they'd be doing this by now... and obviously don't want to ask their moms about it.