As pointed out in the other answers, there are many opinions and many factors to consider.
We put our daughter into daycare when she was three years old, and for her earlier would probably have been better, to get more play time with other children.
My personal impression is that children under one year probably do not get all that much benefit from daycare, though it probably does not hurt either. Between one and two, other children start getting interesting, so daycare starts having a benefit for the child, even if the parents have time for home care. And from two or three onwards, most children really need regular time with other children, and benefit from the (relative) independence and interaction with others they experience at daycare, so daycare is advisable.
With our daughter, when she was two to two and a half year old, she started asking why there were no other children in the playground, or why another child had to leave - that made it clear that she needed company.
So my recommendation would be:
- up to one year: daycare only if required for practical reasons (such as working parents)
- up to two years: depending on development of the child
- older than two or three: definitely, for the social aspect alone
Note that in my opinion even children with lots of playground friends or older siblings profit from daycare, because of the other aspects, such as: learning to interact with adults other than the parents, learning different rules, having to get along with children they may not like so much... and just generally getting a perspective on things different from what the parents provide.
Note: This is about daycare in Germany, which is focused on playing and exploring, not on teaching/learning. In some countries, daycare has more of a pre-school aspect (e.g. "kindergarten" in the USA) - that's a different kettle of fish which I cannot speak to.