I'd look through the link that the individual provided. Age limits to be a federal agent is 37 years of age at time of entry. This is due to mandatory retirement (from primary position) at 57 years of age.The other big requirement is a 4 year degree. NCIS typically likes to see some previous law enforcement, language, or computer forensic skills.
Height and weight requirements vary by agency, I've seen feds over 300lbs. Typically it's later in their career though. Some agencies make you take a physical battery (i.e. 1.5mile run, push ups, sit ups) before you go to their academy - but many do not. I'm not aware of NCIS doing that at this point.
Take a look at www.usajobs.com, and do a series (occupation) search of 1811 - that's Criminal Investigator - and you'll be able to see vacancies posted for various agencies.