A) Does that help delay in nearsightedness worsening?
Some doctors believe that it does. There is a theory that if you wear your glasses (which correct for distance vision) to look at things up close, that you "strain". Straining is just using your eye muscles to overcome a visual deficiency. Using your eye muscles can raise the temperature in your eye, the heat tends to break down eyeball tissue, and your eye can become elongated, which increases your nearsightedness.
B) Is presbyopia inevitable? I'm 45 and I still feel good about seeing everything near without a lens....
Yes, it's inevitable, and you're probably already on your way. For now you can see up close without your glasses because you're nearsighted. The test is to read something like the telephone book at about 18 inches
with your glasses on.
You can't count on being able to read comfortably without glasses forever. Nearsighted corrections are always a
minus value. A presbyopic correction is always a
plus value "added" to the distance power. My guess is that your current distance Rx is about -1.00, and your (unused) presbyopic "add" is about +1.00, making your net near vision power zero. That's why you can see close without correction. Eventually, your "add" power will increase, typically topping out at +2.50 when you reach 60-ish. At that point your net near vision power might well be +1.50 (-1.00 +2.50 = +1.50) (assuming no change in your distance Rx). Then you'll have to decide if you want two pairs of glasses (or more, because the power you'll need for "near" vision will not be the same as the power you'll need for "intermediate" vision. Intermedite is about 20" to 30" or computer distance) or multifocals.
Reading Adds
Finding the correct reading add is a difficult task. Most refraction errors come from improper add power. We will not attempt to discuss a great deal of theory here, but you should know that a patient can comfortably utilize - of their available amplitude of accommodation (the reciprocal of the near point). Amplitude diminishes with age. For example, researchers claim we have at age 10, between 11 and 14 diopters of accommodation amplitude, by age 40 it is between 4.5 and 5.5 (Borish, Vol. 1, 1970 pp. 169-170). It takes +2.50 D of accommodation to focus at 16 inches, normal reading distance. If we only have +5.00 available, it is easy to see why we may need bifocals at age 40. Unfortunately, all people are not the same. Some may need a +1.00 add at 40, while others may require a +1.25.
A fairly simple, but effective way to determine add power is to use the Eggers Chart for near. A rule of thumb that works well, states that at age 40 a +1.00 to +1.25 add will be required. For each 5 years after 40, and additional +.025 is required.