Can you become "mentally" bilingual after 8? Is it possible to learn a second language good enough to be able to think in it the same way we think in our first language after the critical language learning period?
Anecdotally speaking yes, I moved to another country where I couldn't practice my native one and after some years I started thinking in the once foreign one. I am now back to my original country and the process is playing out in reverse with some interesting quirks, I translate words rather than retrieve the original ones, something that at one point I did in the foreign language.

Strictly speaking, my understanding of how speech and language works in the context of thinking, is that word selection is heavily influenced by both internal and external cues, ( this goes in line with how memory and conditioning work), further, models of speech generation ( both internal and external) support the idea of a general store of meaning (semantic meaning, syntactic rules,cadence,etc) generally dispersed around the NC and more locally near speech producing areas and a separate but heavily connected speech generating system (lateral sulcus: Wernicke- Broca Area).

While speaking a foreign language, working memory areas are conceivably recruited ( this is my speculation ) to translate and then produce speech, something that takes a perceivable time and is noticed by native speakers and sometimes the speaker.

With time and due to plasticity, foreign speech networks get strengthened to the point of not needing any more translation and both internal (what you might call thinking) and external speech are more efficiently generated.

Native speech networks can also face extinction due to the competing foreign language and disuse, but thankfully are still available ( what was once learned is easier to re-learn) so retaking or even thinking in both languages is possible.

As for age, there is a marked advantage of being young due to known efficiencies that happen earlier in life as well as possibly being primed for foreign languages. Still,due to how plastic the brain is even into adulthood, you can become mentally bilingual at practically any age but at a different rate.

Sources:


Kemmerer, D. (2014). Cognitive neuroscience of language. Psychology Press.
Gluck, M. A., Mercado, E., &amp; Myers, C. E. (2013). Learning and memory: From brain to behavior. Palgrave Macmillan.