I love to watch them use the cat flap - if they are in the pens from birth, they will attempt the flaps from about 3 weeks or just over 3 weeks. In Milos litter, one of the girls was 1st - she was batting the door with her paw, then popped her head out, that was enought for that day - then gradually got braver until out she popped at just under 3 and a half weeks.
If they come into me when they are older, it takes them a while to master the cat flaps.
Also, the pens have a door (I'll try & find a photo)- found one!
If they are with me from tiny kittens - they learn very quickly their limitations & can gage very well how to get in & out & that they can't get close to the end of the pen or they'll fall out.
Like with Milos lot, I had the door closed until they could manage the cat flap, then after a few days I opened it & it's now open all the time. They wizz up & down the steps & in & out of the pen, but they are only 5 weeks. However if a litter comes in to me at 6 or 7 weeks, I have to be very careful when I open the door.
Strange isn't it.
Bless 'em.
Does any of this post actually make sense?