I doubt it was a felony or you would have not been in the military as you would not have been capable of holding a firearm. If you were in deferred adjudication, it is possible the matter was reduced or even dismissed at the conclusion if you attended all the court hearings. At this point, you really need to find out what your record says.

https://www.dps.texas.gov/administra...oryrecords.htm

If it is a misdemeanor and not otherwise a mandatory exclusion from firearms possession or employment as a peace officer, 12 years of honorable and acceptable military should be sufficient to overcome a youthful indiscretion.

As for the age thing, this may vary by agency and retirement system. The feds have a hiring age minimum for peace officers as a result of their retirement system - as does the state of CA. Local agencies, however, are unable to have a hiring minimum. TX may be different, of course, but do not assume that the standard for one agency might be the standard for all. Per TX state law, there is only a MINIMUM age for certification as a peace officer (18 or 21, depending on other qualification)