Answer by Palaeologus
That's a tough one Bereket. The thing that helped Reagan lighten it up was that it was patently absurd to refer to Mondale as an inexperienced whippersnapper.
You don't want to call your opponent old, unless he's also a young candidate. Even then I'm not sure it's a good idea. In an interview you won't be able control the narrative the way a debater on stage can, and it would be easy for a reporter to make your candidate look bad.
I'd focus on the experience thing and bring it back to whatever you want to attack about his record, eg My opponent is experienced at overspending, overregulating, making sweetheart taxpayer funded deals for his pals, etc. If that's the type of experience you want, he's your guy. Then pivot back to whatever positive platform your candidate is offering in contrast.