I'd guess you take a medium but at 5'6" with shorter legs I find myself right on the edge of small and medium 29er mountain bikes.

As for your question about bike age, you're asking for advice from a guy who rides a 1994 Specialized S-Works M2 on the local trails. Age is not a concern to me. Other than upgrading to an air fork, it's pretty much as Specialized built it. I'll gladly ride a nice older high end bike over newer lower level - the only gripe is that M2 has 26 inch tires and rim brakes. For any trail riding, I'd rather have discs.