From one perspective, the exact opposite is the case. Rising pensions are better for youngster than oldsters. This is simply because they can look forward to many years of rises, whilst an old person will probably die after only a few years of getting them. If you're 40 years shy of pension age*, a 0.5% annual real rise in the state pension increases the value of your retirement wealth by over 25%. This is much more than an 80-year-old will get, who can anticipate only eight years of rising pensions before he dies.
Why, then, does everyone think a rising pension is a bribe to oldies rather than youngsters?
One possibility is that Cameron's pledge is unaffordable and so it'll be reversed before 20-somethings retire. This, though, is doubtful; long-run forecasts for the public finances are subject to considerable uncertainty. And those who see the promise as a bribe aren't arguing that it is merely temporary.
Another possibility is that we discount the future, and so a higher pension is less valuable to youngsters than it is to older folk. However, long-term real interest rates are close to zero. Even if we add to this the probability of death, which is less than 0.5% per year for the under-50s, we still have a low time-discount rate - so low that it could be that pension growth outstrips the discount rate, which would mean youngsters benefit more than oldsters from rising pensions.
Why, then, does everyone seem to take for granted that rising pensions are better for the old than young?
One possibility is that there's a bubble in gilts, which means that gilt yields are no guide to people's actual rate of time preference. Another possibility is that young people are risk-averse, and attach more weight to the risk of dying before pension age than life tables suggest. A third possibility is that Cameron's pledge carries a cost of higher taxes, and these will be borne by today's (and tommorow's) working people rather than by today's pensioners. In this sense, pensioners are getting something for nothing, whereas younger people are paying for their higher future pensions; oddly, though, nobody has translated Cameron's pensions pledge to mean "higher taxes on working-age people."
There is, though, a fourth possibility. It's that the commentariat believes (rightly or wrongly) that young people have very high time discount rates - say because of hyperbolic discounting or some other form of present bias - and so just don't appreciate that rising pensions will benefit them.
I'm not sure which of these is right; they might all be to some degree. My point, though, is simply that the kneejerk view that rising pensions are a bribe to the elderly requires some anciallary assumptions to hold if it is to be correct.
* Yes, the pension age will rise over time, but it's not obvious that Cameron's announcement yesterday represents any new news on this front.
Another thing: altruism complicates things a lot; I'm assuming it away.