802.11ac on AEBS the old flat one

I have an older airport extreme base station. Maybe 4 years old now.
I noticed in my system report that it says capable of 802b/g/n/ac but my new retina iMac is only connected at N.

If I buy the new airport extreme that will give me 802.11 ac. Can i use this older airport to extend my network so it reaches all areas of my house? will it extend the 802.11ac or will it drop it down?

I believe that ALL Macs and wi-fi devices must be able to support 802.11 ac, otherwise they will attempt to fall back to something they can all use.