I am glad more people are considering oldies - I only adopt oldies, I currently have a 15 and 18yo here, and have lost a 15 and 16yo this year (they were both poorly when they came, it hadn't been picked up in their previous home though). IT all really depends on the cat, as i have also lost a 5 and 11 year old. If not already done, I would recommend a full blood test to make sure all his organs are fine, and that way you have a base level.