I feel like that, too!
I got Addie when she was nine months (part of rehoming because the family felt they couldn't give her the attention she deserved, especially with another two or three kids and another on the way). It's not that serious, I guess since I still did get her before she was a year, but I often wonder what she looked like, how she acted and everything else about her. They just look so different when their puppies, and I really miss that. I definitely wouldn't trade my pup for anything, but it still makes me a little sad sometimes to not have had that experience with her.
Plus, I wonder if she ever thinks about and misses her previous family.
It's probably part of the reason why I spoil her soooo much ... although, I would have spoiled her regardless!