Yet Democrats seem to prefer talking about their own candidate’s deficiency instead of the other party’s.

Yes. Because their own candidate's deficiency is costing them the election right now.

If Biden were winning in the polls, this wouldn't be happening. If Biden weren't trailing other Democratic candidates in most states and congressional districts, this wouldn't be happening. If Biden's age wasn't such a huge and important issue even for Democrats, this wouldn't be happening.

But he's losing, and losing by a non-trivial margin. He's down 3.3 in the RCP poll (nationwide, head-to-head) - compared to up 8.7 this time in 2020. And one of the major things that voters consistently say they don't like about Biden is his age. So even though we think it shouldn't the the case, it is the case that Biden's deficiency is far more damaging to his performance among the voters than Trump's deficiency.

And there's a meta-problem here. Typically, the most important way that a campaign emphasizes the opponent's deficiency and minimizes their own candidate's deficiency is by using the candidate to do that. But Biden's deficiency also affects his ability to convey the campaign's message and communicate with voters directly. That's not a hugely essential part of the job of being President, but it is a hugely essential part of being a candidate for President. If Biden can't do it - if he can't do a series of Town Halls and barnstorming tours and live events in the home stretch of the campaign - then that hurts him more than Trump's deficiency does.

Biden is far more likely to lose the election due to his age than Trump is to lose the election due to his felony convictions right now. And Biden's age might prevent the campaign from doing anything to change that dynamic, because it might affect his ability to campaign. Which is why Democrats are more focused at the moment on Biden's age problem than Trump's felony convictions.