I believe the Ocular Hypertension study showed that ocular
hypertensives convert to glaucoma at the rate of about two percent per
year. So if that's all you looked at, then over an average lifespan the
21 year old would be more likely to live long enough to get glaucoma.
But there are lots of other factors--for example, the higher the eye
pressure, the higher the risk, and thickness of the cornea plays a part
too. Also, from what I've read, there is some natural thinning of the
optic nerve fiber caused by aging, so it might happen that the 40 year
old's optic nerve becomes less resistant to pressure over time. Another
thing is that eye pressures often tend to rise as people age anyway.

I'm not a doctor, this is just gathered from various sources I've read.
But my guess is that there are too many factors that influence the
development of glaucoma (some people with higher than what is
considered normal eye pressures will never have any damage, and of
course normal tension glaucoma patients have damage even though they
are not considered to have ocular hypertension) that it would be hard
to predict. One thing, however, I think is that the 21 year old can
pretty much count on better treatments and probably a cure for glaucoma
in his lifetime, although possibly the same is true of someone
currently age 40. I hope so.
-Gudrun