I have a Canon 7D and have located the serial number, how can I checked the manufacture date?
Possibly, but the information to decode date of manufacture doesn't seem to be known outside of the Canon company.
Canon bodies and lenses used to have a date code stamped on them, separate from the serial number. That practice was phased out for some, but not all, products in 2010 or 2011.
According to Wikipedia, the Canon 7D was announced in late 2009, so if the camera was made early in the production run, it's possible (but unlikely) that your camera body has a date code stamped on it. The date code will have the following format,
P is the factory where the product was manufactured: "F" (Fukushima), "U" (Utsonomiya), or "O" (Oita). Apparently, most (if not all) of the lenses have been made at Utsonomiya, and SLR bodies are made at Oita (and formerly Fukushima). Thus, date codes for lenses usually start with "U", and SLR bodies' date codes start with "O" (or "F" until about 1991).
Y is a letter code for year of manufacture, starting with A = 1986, through Z = 2011 (they used the same A-Z letter code for year of manufacture starting with 1960, but used a slightly different format for the rest of the date code then). So if your 7D has a date code, it could only start with X, Y, Z (X being unlikely).
MM is the month of manufacture: 01 = January, ... 12 = December.
## is a 2-digit number that has no apparent meaning for decoding the date of manufacture.
How old is my camera or lens?
Canon EOS products often have date codes stamped onto them. These alphanumeric codes are separate from the numeric serial number and are usually hidden away somewhere - inside the film chamber of most cameras or on the black light baffle on the underside of many lenses. Not all EOS products have this code (for some reason Canon gear built in Taiwan often lacks date coding, and Canon is dropping date codes as of 2010) and those that do often have the code printed in shiny black ink that’s hard to read.
The code looks like UG0205, for example. The first letter represents the name of the factory at which the product was made - often O for Oita (cameras) or U for Utsonomiya (lenses). The second letter is the date code, in which A is the year 1986. The next two digits are the month of manufacture, and the last two digits are apparently internal codes meaningful only to Canon. In the UG0205 example, therefore, my lens was built in Utsonomiya in February 1992.
Canon lens date codes and EF lens chronology. Interestingly, this page indicates that as of 2016, at least some lenses still have this date code stamped on them (contradicting other references that say the date code system was phased out in 2010 or 2011:
The date codes restarted at 'A' again in 2012. This TS-E24 mk2 lens 'UD0406' was purchased in 2015 (box dated May 2015)
It seems that the old date code system is still in use - a TS-E17 UC0903 (Sept. 2014) is consistent with the box date (Oct. 2014)
Canon Camera & Lens Date/Factory Code Explained from ebay.co.uk, written 2006.
At Bryan Carnathan's site, The Digital Picture, the article Determining the Age of a Canon Lens Using Serial Numbers and Date Codes shows a table for converting the first two digits of the serial number of a Canon lens. However, regarding the applicability of this decoding to DSLRs, the article notes that
Canon EOS DSLR camera body serial numbers, at least for 2013, do not follow this chart. Also please note that future dates shown in the table are predictions/expectations.