Dr. Malcolm M. Manners, Professor of Citrus Science at Florida Southern College and consultant to the Florida citrus industry, had your answer posted a few years ago, but it has since been removed. He said that Grapefruit Trees, like other citrus, don't measure maturity in years like people. Instead, they measure it in nodes.
Nodes, you probably know, are the nodules up and down a stem where you see leaves growing.
Grapefruit trees have to get very, very tall to get to the point where they have enough nodes to reach blooming size. I've seen some reports where they are not tall enough at 15 feet. In other words, if you are growing this on the windowsill, or even in your average greenhouse, and you want to pick Grapefruit off your own tree for breakfast, don't hold your breath.
The high-profile Dr Manners has straightened out many a southern back yard grapefruit grower. Here's his q&a session in Southern Living Magazine:
Alas, that will only help you once your tree finally blooms.
I posed your question to Dr Manners. Yes, I really, really did. And Dr Manners answered me. He said that the number of nodes is indeed the deciding factor as to when you'll be picking Grapefruits.
'It varies from variety to variety, so no one has actually ever published exact numbers," he said. "With Grapefruit, I'd say somewhere in the 8 to10 feet high area.'
Remember, that's a rough guess, albeit from a world famous citrus celebrity. Odds are, not even you know now what 'variety' of Grapefruit seed you sowed 5 years ago.
But what the heck. May as well be prepared. Dr Manners gives a short lesson to Southern Living describing how to tell, perhaps years from now, when your Grapefruits are ready to pick:
"Malcolm Manners recommends testing one or two for flavor before you pick a lot off your tree. 'Palates vary, so your citrus is ripe exactly when it tastes ripe to you.' Also, he explains, young trees--especially grapefruit and navel oranges--may bear only dry, pithy fruit for the first year or two. 'After the third or fourth year, most citrus trees can be expected to bear generous quantities of good fruit, and they'll keep producing at this level for 25 years or more.' Tip: Remove any fruit from a newly planted tree so it will put all its energy into growing. You'll raise a better, stronger tree by sacrificing this first little crop."
Start counting nodes.
How important are those nodes? Well, if you lopped off the top of a blooming Grapefruit Tree and rooted it, the new 'tree' would grow Grapefruit. It still 'thinks' it's growing at the top of the old tree. The node knows...not.
Quite fascinating, if you think about it. Patience, my friend. Good things come to those who wait.