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This could be a fertilization problem: if the nitrogen supply is too high or the phosphorus supply is too low in their soil, citrus trees will not flower. It could also be a pruning problem.

These articles are about oranges but are relevant to all citrus trees:

https://www.gardeningknowhow.com/edible/fruits/oranges/no-fruit-on-orange-trees.htm

https://www.gardeningknowhow.com/edible/fruits/oranges/pruning-orange-trees.htm

One possibility is that some varieties will bear in alternate years, with a heavy crop in one year and a ligt crop in the next year. Cold temperatures that kill blossoms, insufficient fertilizer, and watering problems can all cause poor fruit production.

https://www.gardeningknowhow.com/edible/fruits/oranges/no-fruit-on-orange-trees.htm

You'll have to look up the nocturnal beasts in your area. Where I live in Michigan, prime suspects are opossums and raccoons. Skunks don't climb trees but will eat fruit that has fallen.

They are, likely, self fertile. Just having another tree around will increase pollination. They will be able to pollinate with each other if you plant the same trees.

Once you see flowering, it is an early sign that fruit will soon follow. This could be 1 to 3 years, but it is usually fairly soon after the first flowers.

You can give it a boost by giving it a citrus fertilizer. This will give it the nutrients that it needs to properly produce fruit.

This article will give you more information on the care of these: https://www.gardeningknowhow.com/edible/fruits/tangelo/tangelo-tree-information.htm

Generally, 40 or 50 years will be expected. They tend to decline, suddenly, seemingly. It will be time to look into replacing the tree.