DD2 is 13 weeks' old, sleeping in my bedroom and about to move from a moses' basket into a cot.
My 3 year old DD1 sleeps in a single bed in her own bedroom with the door closed. She doesn't like sleeping on her own and mentions it most nights when we put her to bed. I feel really sorry for her. She sleeps through most nights but sometimes wakes us up and I wonder if she's lonely. 
When we visit friends and share a room DD1 sometimes gets over-excited the first night but is fine by night 2.
I think DD1 would really enjoy sharing her bedroom but as she can get out of her bed I'm worried that she's too young to share with a vulnerable baby. She might try to poke her, give her inappropriate toys which she could choke on or unintentionally smother her. During the day I don't yet leave them alone together for more than a minute; I'll take at least one of them with me if I need to do something in a different room. DD1 is very affectionate but often makes DD2 cry with her over-enthusiastic cuddles.
Also, I was hoping to breastfeed until about 8 months (going back to work at 9 months) and it's been nice doing night-feeds in my own bed. I'm not keen on the idea of going to DD1's bedroom for 2 feeds each night.
From what age do you think our baby would be safe with our toddler. What experiences have you all had?