In a new bath room, much of the plumbing is plastic (PVC or ABS). in an older building it would be copper and cast iron, which will block more of the signal. Bathrooms also tend to have more wiring then other rooms which could block more of the signal. If the walls are tiled, they not only have the tile, but usually have a heavier cement based backer board which will block more of the signal then regular dry wall. Also don't forget the tub. A large cast iron tub would definitely block part of the signal. A fiberglass one would not block as much.
Finally as for the mirror, the silver backing can be expected to block the signal. On older mirrors the coating was actually silver which is an excellent conductor, meaning it would block the signal. Newer mirrors use a thinner silver colored paint usually with at least some aluminum in mix. It will still block at least some of the signal, but not as much as the thicker silver.
In summary, a new bathroom will not block as much of the signal as an older one will.
1I've seen a similar issue a while ago... Is it double-sided mirrors or single sided? – r0ca – 2010-08-04T21:01:59.477
1Yes, everyone, there is a bathroom between the mirrors. – Ryan C. Thompson – 2010-08-05T04:05:34.547
7Definitly, we need to send this to MYTHBUSTERS! – r0ca – 2010-08-05T12:51:14.847
Apart from amoured concrete mentioned below, there are other reasons to get a bad signal too: 1. another router sending on the same channel (if the other router uses channel 1, use 6) 2. water in pipes 3. bluetooth, microwave ovens, a phone using 2.4 GHz band. Try to move the router 1 meter left, right, up, down and check the signal. – ott-- – 2011-12-26T17:38:47.523