You can graft as a sapling but I highly recommend allowing the tree to get large before top working. If you Graft a small tree with mature scions, it will flower within the next year. I’ve grafted hundreds of mango saplings using epicotyl grafts and I’ve also grafted many year old saplings and they will flower within a year unless you bring them in during cold weather or if you Graft with seedling scions.
When these young trees flower, it significantly slows down the rate of growth because energy is put into flowering and not into vegetative growth. The leaves are the trees solar panels and if the tree didn’t waste it’s energy on flowering, it would have approximately twice as many leaves. There is then the additive effect that each additional leaf it would have had, could capture that much more sun energy.
This process then repeats every winter and the tree that was grafted when small will again bloom and the seedling that was not grafted will again grow vegetatively once the weather is sufficiently warm enough for growth.
Here are a couple seedlings that were grafted as a young sapling:
CAC graft on White Pirie seedling
Sweet Tart graft on Kent seedling
Same grafted sweet Tart on Kent rootstock vs Kent Seedling that was not grafted. The trees are the same age but look at the difference in height. Don’t count height of the flower stock on the grafted tree because it will be removed anyways. The ungrafted tree is twice the height of the grafted tree.
In case anyone is wondering, yes, these roughly 12 inch tall trees grafted with mature scions will attempt to hold fruit to maturity but the fruit will be tiny, may not fully ripen and will severely stunt your tree. Here is another Sweet Tart grafted onto Fruit Cocktail seedling holding several tiny fruit.
It is best to graft trees when they have reached a mature fruiting size, ideally about 4+ feet tall with several well developed scaffold branches.
The issue with this technique is that you will have to know how to top work a mature tree and you will need to perform multiple grafts and will require multiple scions.
When grafting saplings, you only need a single scion and if you perform epicotyl grafts, you can get near 100% takes.
Simon