My sister got glasses for nearsightedness at 25. Is it true that if you get glasses when you are older that your prescription isn't likely to get that strong? Is it possible to estimate how strong someone's glasses would get if they had -1.50 at age 25?
Its likely to stay the same (-1.50) as an adult's eyes are less flexible than a teenager/child so the eyeball won't elongate as much. Also its possible for the power to decrease because when you get older your eyes tend to go in the other direction and become farsighted!
If she needs glasses she should use them when she needs to when clear distance vision is required and take them off whenever she does close work like reading. She does not need to wear them all the time as her prescription is low.
The answer is an unknown in all cases. It is usual to develop a prescription at a much younger age, developing a prescription at the age of 25 is not typical. However, it is quite common for people to have such a prescription and not wear correction for it initially. I have always had a prescription of +0.25 in both eyes, at the age of 27 my right eye changed to -0.75 / -0.50, it has remained the same over the last 3 years. The cause and effect are unknown and this is what I do for a living, so believe me when I say I had every bit of my eyes anatomy checked.
You could not estimate the size of prescription in the future, not for anyone. A retinal detachment could change that prescription to -10 overnight, keratoconus will change the prescription continuously until treated, pregnancy and breast feeding will alter the nature of the prescription as could any form of drug intake.
All one could say is that so long as the stimulus for this prescriptive change is not disease orientated then is is perhaps unlikely that the prescription will change as much as it may have done in your teens.
**At the age of 25, she is not presbyopic and hence should wear her glasses to read as well. ''Lazy'' in this case should not be understood in it's normal optical usage (amblyopia), rather as a misguided lay verb. The muscles in your eye do not become ''lazy'', the brain does - they do not become lazy in later life either - the lens loses elasticity causing the need for reading glasses.