Most people would know the age of the aircraft they were in if it was placarded on the forward bulkhead. Cases in point are the MD80's currently operated by airlines like American. 30 years old and approaching 100,000 hours. I am currently flying a 38 year old Lear 35 with over 25,000 hours on it. And it flying just as well as the last 35 built.
As long as they are clean and well maintained (including paint and interior) people don't know or care how old they are.
Your biggest concern is making sure the aircraft will fly on a regular basis. To ensure that, you will need to find an aircraft that the market will support. A G650 out of Podunk, KY will not fly, as that is most likely a Seneca/King Air market. Each charter market has its own dynamic. One of the best way to find what it is, is to search a website like Flightaware.com and see what is going into and out of the market.
Two important factors to consider are reliability and support. While modern manufactured aircraft are reliable, as they get older the reliability decreases, some models faster than others. The biggest factor is manufacturer support. Check out the manufacturer reviews in Aviation Industry News. Some like Gulfstream get top notch reviews. This becomes more important as the aircraft gets older and the model is no longer produced.
Good luck.