I recently grafted a cutting of an older mango tree to a younger root stock (1-2 years old). I am curious what's going to happen. Will the tree fruit earlier?
Almost all scions are from older trees than the rootstock, except when topworking an old tree to change variety.
Then again, scions are generally the "less than 1 year old" wood from the older tree, for best success when grafting.
It will probably make no difference in time of fruiting. In some cases of precocious fruit set (within 1 or 2 years of grafting, but I'm familiar with apples, not mangos) it is advisable to remove all the fruit until the graft union is stronger.