How Long Does It Take for the Beta Cells to Be Destroyed Before the Symptoms of Type 1 Diabetes Appear?

Wow, that's late onset! But it does happen. I don't know if anyone really knows how long it takes beta cells to be destroyed, because people are usually only diagnosed once the symptoms are advanced. Some Type 1's experience what they call a "honeymoon period" for the first few months to a year, when there are still some functioning beta cells that haven't been destroyed yet. But they die off and you make little to no insulin from then on. (I didn't have that, it was full tilt right from the start)

Since you're athletic, you may want to consider an insulin pump, which gives you much more control over insulin delivery. You can dial it down when you exercise to help reduce "lows". Assuming you're not involved in contact sports like football, it would probably be an excellent choice for you.