Katrina from WorldNomads.com here.
Unfortunately it is not possible to upgrade or buy cover with the plans we offer if you are over the age limit our underwriters can cover. This is a requirement from our underwriters as our plans are designed and priced for a single age limit.
The plans we offer do have different age limits depending on your country of residence (which you don't mention) These are valid as of 3 September 2012:
USA residents: under 67 years old
Canadian residents: under 60 years old
Australia & New Zealand residents: Under 66 years old
UK & Ireland: Under 60 years old
All other countries: under 66 years old
For others reading this answer: All travel insurance policies are different, and the plans we offer differ depending on your country of residence. Please do read the terms and conditions and the full description of cover (policy wording) for full details on who can be covered: http://www.worldnomads.com/travel-insurance/prices-and-benefits.aspx
If you have any questions, please do contact our customer services team directly: http://worldnomads.com/contactus.aspx
Safe travels, about 6 years ago