6 months.. once she was active and alert, she didn't want to go to sleep. She thought if she was around us, then it was playtime. She's now 18 months and I can proudly say that I will tell her it's time to go night night and she will go lay down in her toddler bed! She doesn't get up in the middle of the night, but she's had a cough the last couple nights so when I hear her, I go snatch her up and sneak her into bed with us.
I'm thinking of skipping the crib and just waiting out till it's time for a toddler bed... Only thing is what if we decided to transition earlier