In pictures, it's really hard to tell. But in person, I think I have a general idea. Charlie is 27 months and the baby-ness in his face is disappearing. I've noticed that kids just 3 months younger look younger than him, so I use him as a guideline.

With babies, sometimes it's hard to tell because some are so chubby! But I guess it's easy to tell based on whether or not they're sitting up or crawling!

Can you tell?