My expertise isn’t really in this area but I’ve been doing a bit of research as tonight’s live chat was so quiet! From what I can find most studies addressing this look at younger vs. older children within the same year group, e.g. 5-5.5 vs 5.5-6, rather than starting at 4 vs 6 for example. It’s hard to compare different countries that start at different ages as there are so many factors other than age that differ between the samples.
Regardless of the age at which children start school their age needs to be taken into account in assessments of their ability . This has been explained beautifully and clearly in a blog post by Professor Dorothy Bishop at Oxford. I highly recommend it – focused on summer born children who start school at 4. http://deevybee.blogspot.co.uk/2015/05/great-expectations-our-early.html
It seems to me that we don’t really know whether starting school young actively causes harm but evidence from international studies such as PISA suggests that, either way, starting school later doesn’t -particularly when a later start to formal education is combined with high quality play-based kindergarten.
I don’t really have anything to add to the other answers but I would point out that we’d need to define what we mean by ‘formal education’ to answer this question.
My hunch (with no supporting evidence!) is that getting children to sit at desks and engage in the type of activities normally associated with secondary schools from a very early age is likely to be harmful. My rationale is that a critically important part of learning involves physical interactions with the environment and learning how objects behave (this notion is somewhat formalised in the writings of Piaget, 1954 and theories of ’embodied cognition’) so removing such opportunities is potentially detrimental to children.
Conversely, providing opportunities for children to socialise with other children and have access to environments that might otherwise be unavailable (e.g. sandpits) – especially for children from low socioeconomic groups – might be extremely beneficial.