Measure the tree from the bud union to the top of the tree's canopy. This is where the Skaggs Bonanza was grafted, or attached, onto the dwarf rootstock. It appears as a lump on the main trunk of the tree a few inches above the surface of the soil.
Estimate the age of the tree according to its measurements. Dwarf citrus trees grow at a moderate rate. This is defined as between 13 to 24 inches per year, according to the Arbor Day Foundation's website. A tree that is 2 feet tall is probably around 2 or 3 years old.
Add a year to the Bonanza dwarf tree's estimated age. This is because most dwarf citrus trees are already a year, if not slightly older, by the time they are grafted onto the rootstock.