'Early intervention' can be interpreted in a number of ways.
It would be useful to be able to give a slick definition of early intervention, but that is not easy due to the complexities of how children learn. How and when decisions might be taken depends on a number of factors.
What is Early Intervention?
Moira Leslie and Greg McMillan in the 'Early Intervention Handbook', which was published by the City of Edinburgh Council, give a list of features of early intervention that go some way to providing a description.