I tried out different applications from Fdroid. But some I rather have no clue if they work as they should with e/ or if they can do any haresment to the system. I noticed some are very old. Even older than the e/-OSM.
As the app-lounge often is full of Fdroid apps should I then blindly trust what come from there. As e/ foundation is a trusted community ?
Regain your privacy! Adopt /e/ the unGoogled mobile OS and online services
I always check when the package has been updated. F-Droid is much more trustworthy source than Play-Store, but remember to make some research before installing anything. 
Can you give an example, just for us to get a feel of what you expect to happen?
F-Droid builds all Apps it offers from the available open source code. Furthermore it lists what it calls Anti-Features for every App, so that the user can make an informed decision.
The date of the last update could be factored into a decision to install an App or not, especially if there are several Apps from which to choose, but it shouldn’t be too important. Sometimes an App just works. Period.
For the sake of argument I could say I put more trust in Apps which don’t get updated too often, because they apparently need less fixing 
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No, not blindly, you are perfectly free to ruin your /e/OS system by adding random apps without thought. App Lounge does not offer to scrub undesirable apps.
However App Lounge does include content from Exodus. Use the Exodus content to exercise your judgement on what to install.
I also have the Exodus app installed on my phone as a further source of information.
As other people have said here you should do your research on applications before installing them. But having said that most applications that come from f-droid will do you no harm as most of them don’t have trackers in them and most of the time if they do it will be reasonable ones like sentry and I’ve never seen one with malicious intent. The age of the apps most of the time does not affect what they do and it would most likely in the worst case scenario prevent the app from working properly. And the only time i’d definitely suggest not using an outdated app even if there is no other option is if it needs things like root, adb or device admin permissions. Which those are the types of apps that need to be researched the most cause even if they don’t have malicious intent all it takes is a bug to mess up your device
This might be instructive: