all my 15 mango trees were grown from seed

Grafted mangos produce fruit quite quickly. The graft is actually the age of the parent tree it came from so it's usually got a number of years behind it. But trees from seed take a lot longer. You could be waiting 8 or more years, depending on your climate.

but a tree grown from seed will produce fruit right?

In normal circumstances it will, even if it takes a while.

Mine just flowered for the first time. Was seed grown, and is about 10 years old. I can't wait to see the fruit.

Most common mango varieties are MONO and will not grow true from seed. They will probably eventually fruit but 99% of the time the fruit or tree's characteristics will be inferior to it's parent. However in most cases they should produce good tasting even if fibrous fruit, but reliable production could be more iffy.

I have an elderly neighbor with 5-6 large mango trees all grown from seed and he enjoys the fruits he gets, when he gets it as they often don't bear in many years.

The small fruited Mangoes,like Manila and Aultalfo that you buy in markets are true. Was it one of those?

My first ever Manila took 3 years to fruit and it must have been about a year old or a bit more when I bought it.

I have an Aultalfo thats 3 and now in ground- first two in a pot- and I would not expect fruit for at least another 2. But,hey,I'm in a cool Med climate. Florida times seems to cut everything in half or more.