25 years by the model year or year it was made??
85 made in 84 asd so on.....

 

By the model year. That's really the reasonably way to do it.

http://www.txdps.state.tx.us/vi/insp.../item_insp.asp
Which vehicles are required to have the emissions test?
Vehicles registered in designated counties. Designated counties include: Brazoria, Fort Bend, Galveston, Harris, Montgomery, Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Rockwall, Tarrant, Travis, Williamson and El Paso counties.
Gasoline powered vehicles. Diesel powered vehicles and motorcycles are exempt from emissions testing, but are still required to have the annual safety inspection.
Vehicles 2 through 24 years old OR upon expiration of the two-year initial inspection sticker.