I can only relate how we transitioned our two kids, which started when they started to roll (around 4-5 months). My two were very different because of living arrangements, but the philosophy was the same.
For both kids, I had the crib set up in their room and a separate bed (cradle or mini crib) in my room, and neither ever napped in my bed unless I was napping with them. I started with daytime naps, and when they slept relatively well, they started the night in their crib in their room. This happened around 1 year, and while they nursed during the night, I would pull them into my bed when they'd wake to eat and leave them in bed with me.
Around 15-16 months, I started putting them back in their crib after they nursed, and I did this until I night-weaned between 18-20 months.
That being said, my 6.5yo son still crawls into bed with Daddy in the middle of each night and sleeps with him on the weekends. My 3.5yo daughter typically ends up in my bed around 5ish. It's very understandable and natural for a child to want to sleep with someone. After all, don't adults sleep with someone (usually)?