The lifetime depends on how loud they're played and for how long. 20 years may be pushing it. The fluid can be replaced but you have to disassemble the tweeter and put in the right amount. Too much and she'll blow. Too little and she'll ignite. Axiom uses a precision-calibrated pump to get the perfect amount.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated