Cosleeping and Bedsharing can be two very different animals. Cosleeping doesn't have to be bedsharing but bedsharing is always cosleeping.
The best place for a baby to sleep is either in the same bed (bedsharing) or same room (cosleeping) as the mother. Having spent 9-10 months inside their mother, near their warmth, breathing, and heartbeat, it can be a very rough transition to be away from that. Listening and feeling the mother's breathing and heartbeat regulates the baby's heartbeat and breathing, lowers the risk of SIDS, helps establish a better breastfeeding on demand relationship, and can help both of you sleep better at night.
I started bedsharing with my daughter when she was 2 weeks old. We really tried to have her in the crib because that is what "normal" parents did, and eventually I couldn't take the lack of sleep and the crying any longer. I pulled her into bed with my husband and me and never looked back.
My daughter turns four next month and we still bedshare most of the time. Last summer, when I was was pregnant with our son, she had just started sleeping in her own room, which was her own choice, and when I started having issues with my pregnancy, she came back with us. When we lost the baby, I kept putting off getting her back to her own bed. None of us were interested in her leaving. She was a huge comfort when we needed her most.
Now that she is almost four, she wants to be her in own bed again, but doesn't want to leave our room. So, we have a compromise. Her mattress is on the floor right next to our bed, and she has free reign on which bed she wants to sleep in at night. Some nights she will sleep in her bed all night, others she comes into our bed in the middle of the night, and nights which are fewer and fewer she sleeps all night with us all night.
Many would tell me that she is dependent on us to sleep, and it makes me laugh. The truth is, it is the opposite! Because of our losses and things that we have been through, I feel like I need her close more than she needs me close. Loss changes a person, whether it's a baby, your sister, mother, father, cousin. It changes it your perspective on life, and since our losses, my mind goes to places that make me uncomfortable. I worry all the time about losing my daughter. Having her in the same room with me, even if it isn't the same bed all the time, comforts me. I think she knows that and hasn't chosen to go back to her own room because of it.
Many families have different reasons for bedsharing or cosleeping past infancy, one of the more popular ones in the natural community is that they are still nursing. Just because your child is older than a year or two or three doesn't mean they have to be in their own bed in their own room, and that's okay.
The benefits of cosleeping and bedsharing don't end at a certain time. It is the same as nursing. Your child doesn't magically become "too old" to stay in your room or your bed. Every family feels that age differently. Some stop wanting to sleep with their children when they start to flail around at night as inevitably all toddlers do, others like their children to be out of their bed by the time they eat solid foods. Still more don't like to have their children with them at all.
Cosleeping and bedsharing both have amazing benefits, but again, it's a choice for your family. If your family chooses either of those, it is good to know that you can choose when to stop. My daughter is so independent and strong, and personally I feel that knowing I am still just an arm's reach away helps that. I think that growing up with that comfort helped her to feel more confident in herself. She isn't going to always need me just because she is still in our bed sometimes. She isn't going to be terrified to go to her own room when she is ready. She won't be in our bed forever like some believe.
You can keep your child in your room and your bed even if you aren't nursing any longer. You can keep them in your room when they talk and walk and are their own person.
There isn't an age where it is unacceptable. This is your family. Make the choices you need to make to raise happy, healthy, independent children. For us, that is keeping her in our room just for peace of mind, though I know that those days are slowly coming to a close.