The simplest way of boosting documents by age is by applying a reverse sort on age. If that's possible, then you're done!
In other cases where what you're after is a boost and not a sort, then read on!
The general idea here is to boost more recent documents and penalize older documents.
There are a number of ways to boost using functionqueries. One way is: recip + linear, where recip computes an age-based score, and linear is used to boost it.
The dismax query parser provides and easy way to apply boost functions. For example:
Another way is recip + ms:
Try a couple variations. Your mileage may vary.