good shot! Not so much of these buses left in Aachen.
During my 3 visitng there I observed following

1999 - almost every bus is such
2005 - half by half (these/newer units)
2009 - just a few

Люблю такие. На сегодняшний день - в Ахене таких почти нет.
Я там бывал 3 раза. с каждым разом всё меньше:

в 1999ом - почти все такие. даже более старые модели попадались
в 2005го - серединка на половинку, и такие и поновее
в 2009ом - очнеь мало таких.

Очень радует реакция водителя, всегда бы так :)

На фоне - университетская клиника.

Where are those buses gone after service in Aachen?
Куда эти машины отправили после работы в Ахене?

Didn'they go to Russia, to Turkey, maybe to Romania/Bulgaria, or directly to recycling?
Не в Россию ли случайно? Не в Турцию ли, не в Румынию/Болгарию, или прямо на порезку.

I'm interested since I've been in Aachen from three times.
Интересуюсь т.к. был в Ахене трижды.

Это должно быть https://fotobus.msk.ru/vehicle/1172095

The main reason for rapid fleet renewal in Aachen is probably that an environmental zone was introduced in Aachen's city center in 2016. Such old Euro 0 vehicles would no longer have been allowed to drive there without an exemption.

As part of the switch to alternative drives (e.g. hybrid/electric buses), buses in Germany are now rarely used by municipal transport companies for longer than 10-12 years. Sometimes old vehicles have to be scrapped due to subsidies for new purchases. Otherwise they are sold through bus dealers in every conceivable country like Russia, Bulgaria, Greece or even Africa or were used for spare parts. But sometimes the condition was simply too bad. Nobody will buy rusty buses with a million kilometers on them anymore. Many German Standard 2 buses have been sold to the Dutch recycler "Verhaegh Trading" in Winterswijk in recent years, because no one buys them anymore, except perhaps for museum preservation.

The O405N from ASEAG had been mainly exported to Poland (PKS Gdansk), where they're also replaced in the last few years by new vehicles funded by the EU.

O405N and similar vehicle types are unfortunatelly no longer permitted in most German regions, even for school transport in the furthest village, because in many cases new Euro 6 buses are mandatory after new tenders...

With the exception of a few buses that still exist, they have almost died out on German roads in recent years. Even Euro 3 buses like the MAN A20/A21 or Citaro 1st generation are becoming increasingly rare to find anywhere.