I believe the very last ones of those shut down early would have been scheduled for shut down by 2020, so in that case about 9 years earlier than their planned lifetime. Some were shut down mere months before the planned date. Their average age in 2020 would have been around 45 years. They were shut down at ~35 instead.

Younger reactors Germany left running until they also reached around 35 years.

I highly doubt german reactors were designed to only last 35 years. Most gen II light water reactors in the US are expected to operate for 60-80 years.

Edit: ah i reread and see what you meant but my point still stands that 45 years is abnormally short for the type of reactors they had

That's also not what I said. Germany was aiming for 45 years initially (likely planning for overhauls + recertification then, rather than shutdown). Instead they shut down at 35.

However if left as is, all of those shut down in 2011 would have been shut down by ~2020 anyways.

Many reactors can be updated to last longer than their initial design lifetime. This is usually far cheaper than building a new reactor. I expect that if the political environment in germany was more conducive to nuclear, that is what they would have done.

No, they could all have gotten 20 year extensions and operate until 2040. Nuclear reactors should be run for as long as possible because they cost so much to build but are very cheap to operate.

Shutting down reactors that age is very stupid if their is nothing wrong with them. Reactors are commonly being certified for 60 to even 80 years.

And the (original) certification itself isn't all that important:

You can check what needs to be fixed with them now (if anything) and do the renovations to keep them working. As long as the basic design is still considered save today, and as long as maintenance and running costs are well below the revenue you make.

The biggest expense in nuclear power is building them. And a really big part of that exploding cost is in all the dark rituals you have to engage in to placate public opinion. (Like excessively long safety reviews and whatnot.)

If you take an existing nuclear reactor, the status quo works in your favour. Even in the unlikely scenario where your renovation essentially replaces the whole thing (so from an engineering point of view, you might as well build it from scratch), renovation might still be the wise choice exactly because of status quo bias in the population.