fixing the age calculation and return early, if condition is not satisfied

There have been two issues:
1. The variable `$msg` was build, using format string function and translations, even though the post was not aged  and the `$content` variable was not changed.
2. The age calculation was wrong. It only used the year value for it's calculation, rather than the full date. So a post from 2013-12-31 would have been shown as aged after only a day (2014-01-01) using only the year value for the calculation. The new version uses the current timestamp and the post timestamp to calculate the difference as a floating point number. It than uses the rounded down vaule of the fraction.