My baby is one and my oldest is three.
My youngest fell asleep on the way to our photo destination, so I had to wake him when we got there. Which meant that he didn't have his usual easy-going smile on his face the whole time. =) He started walking just a couple days after we took these. I'm kinda' thankful that he wasn't walking quite yet when we did his portraits, because it would have been a bit trickier to keep him still for good shots!
And my big boy turned three this year. He was fighting some major chapped lips when we shot these, so his smiles are kind of funny - he's trying to not smile too big to protect his poor lips! There's always something, though, and I really hate to put off photos waiting for that "perfect" timing. May as well capture your kids as they are - that's part of the charm of kids' portraits. =)