one can argue about whether or not alonso may be a reinforcement, but one thing is sure; his age is NOT a problem. I mean, he turns 27, that's probably the peak of everyone's career (statistically speaking). And considering that most players (who do good) play until they are 31-33, that leaves us another 5 years, which is like the perfect situation. He plays as CM, and he'd have to organize our midfield. That's a thing some 17-yr-old wonder kid from Brazil CANNOT do.
People saying 20 mio for him is too much are right imo, because with 20 mio, we could aim for other people too (i'm thinking about aquilani, or maybe someone else). BUT (as always, there's a 'but') xabi alonso has proved that he could play in spain (on a more technical level) as well as in england (on a more physical level) and he may be just the thing we need in our midfield.
Anyway, we can argue about him a lot, but his age surely is NOT a problem.