Only approximately 45% of clients support IPv6. Clients that don't support IPv6 can't talk to IPv6 servers. Depending on your target market, that might be as high as 70% (India, France) or as low as 0.2% (several african countries). Today most of these devices will have some form of IPv4 connectivity, though it's often through NAT, which is slower and problematic for P2P like games.

https://www.google.com/intl/en/ipv6/statistics.html#tab=per-...