Most mini dachshunds live 10-15 years. About the only way to get an educated guess about his age would be to have the vet look at his teeth.
There are a lot of great books and magazines about dachshunds that you can find at any book store or pet store that will give you more information.
Good luck
Utahmomma
mom to 2 dachshunds and one more on the way
In general, smaller dogs live longer than larger dogs. However, no breed comes with a particular expiration date. :-) Your doxie is a senior dog, but not ancient. You could well have several more years with him if he currently has no major health issues. There's just no telling.