Ironhide is the Autobots' resident weapons specialist and Optimus Prime's old friend. He is old because he is hard. He's probably had enough replacement parts to build a small army, but Ironhide doggedly refuses to roll over and die for anyone. He'll fight to the bitter end and take down more than his fair share of Decepticons in the process.
Ironhide doesn't always see optic to optic with Prime, despite their friendship. He's perhaps the more practical of the two, and more than a little trigger happy, so he can be more willing to push the Autobot code to the limit if he thinks it will get the job done faster.