I have a 11year old modem. However from my knowledge it isn't giving me problems.
I ran an ipv6 test I got "10/10" for for your IPv4 stability and readiness and 0/10 for IPv6. I then tried running it w/o my router wondering if that was a problem and i got 7/10, 0/10 to my surprise.
Anyways it says "Your DNS server (possibly run by your ISP) appears to have IPv6 Internet access" on both runs so i am wondering, is my modem the problem? I know my 'new' router (i think its a 4yr model but apparently solid) doesnt support it with current firmware and my ISP does support ipv6. I cant use ipv6 cause of my router and i might have to request ipv6 support from my ISP but before i do does modems affect it?