I think the age thing before going on the Calling is just an average. Sort of like the life span in real life. We've got an average life span, but you never know how long you'll actually get.
Some GWs will not live very long after becoming a GW. Some will live a bit longer than those 30 years. Most will get their Calling between 10-30 years after becoming a GW.
Joining during the Blight would work as a risk factor; the average time before the calling will be shorter than when there's not a Blight, with more GWs going on their Calling earlier, but some GWs will still go on for a significant time after their joining and maybe even get their 30 years (or more!). And others will only be Wardens very briefly before their Calling sets in.
At least that's how I view it.