We have taken the leap and moved Eli into a "big boy bed" and he loves it!!! He is almost 21 months old and we weren't quite sure how he would take to the new bed, but it's been great! We moved him into a twin bed because that's what we did with Jacob. However, since he's not as tall as Jacob was at this age, we just put the box spring and mattress on the floor rather than putting the frame under it. He can climb in and out of it perfectly, it's just the right height for him.
The first night (Tuesday) was wonderful, he loved the idea of having his own big bed like Jacob's and went straight to sleep and never got up. He didn't even get out of bed when he woke up in the morning, he just sat there singing until I came and got him!! Since then, he still hasn't gotten out of bed! We do have a gate at the top of the stairs in case he decides to make any nighttime treks thru the house, he would only be able to come to our room because the other upstairs doors are closed. I think he feels like a big boy now - like Jacob!!
Notice the rails spanning the entire length of the bed (yes, I'm aware they're different, they were bought at different times for Jacob a couple of years ago). We discovered with Jacob that if we just had the one rail on the bed starting a little way from the head of the bed and then only going about 3/4 of the way to the foot of the bed, Jacob could still manage to fall out of the bed most nights..... So, we ended up with 2 rails all the way down the side and only an opening at the foot of the bed to prevent him from falling out. So, instead of having Eli falling out, we just decided to put both rails on again.
So sad when you realize they are growing up, but it's also so heart-warming!!