The upcoming Ubuntu 14.04 will be the next in Canonical’s biannual long term support release, which now has an extend support schedule on both desktop and server versions. Possibly related to this, the standard desktop releases now have a shorter support period, now only lasting nine months. The first desktop to reach this limit is Ubuntu 13.04, which ends its support phase today.
It’s not the end for people with 13.04 though – it’s easy to update to 13.10 using the inbuilt update software, and from there you’ll be able to upgrade further to 14.04 when it’s released. Otherwise, there’s nothing specifically stopping you from using 13.04, however it may become more and more insecure.