Why is it failing after 7 days? it should be running without any issues, but however that is not a viable solution. In CRM Online any custom code will run for 2 mins post which it will be errored out, so the age calculation may not succeed.
You can have a calculated field if you cannot have an EXE running.
If my response answered your question, please "mark the response as an answer" and also "vote as helpful". Regards, Hari. www.crm2011byhari.blogspot.com