LoyceV's guide is a very good start.
There's also an useful thread from xtraelv: Discussion about acceptable and unacceptable behavior. Community values. DT (see also his second post)
In fact, DT is only acting if there's (coordinated) abuse ongoing. Abuse against DT values / DT standards.
Regarding trust abuse:
We had a case some years ago, when some accounts from the turkish section tried to manipulate their way into DT with friends or even alts. The accounts didn't deserve a positive trust and it was very clear that the accounts were trying to cheat their way into DT.
I've lost track about all the threads but here are some:
https://bitcointalksearch.org/topic/revolution-of-bitcointalkorg-v-5247155
https://bitcointalksearch.org/topic/turkish-merit-source-selling-accounts-and-nepotism-on-local-5219960
The main discussion happened in the dedicated trust topic: https://bitcointalksearch.org/topic/defaulttrust-changes-5095156 (I don't know which page exactly, maybe someone knows about it)
There have been very long discussions about it. The abuse was very clear and some accounts were red tagged. Receiving a red tag is only for serious offenses. And the case some years ago was very serious.
Today, the tools like from LoyceV are very advanced to detect trust abuse very early to avoid escalations.
Often it's enough when the abusers are distrusted (added ~ to the trust list), meaning they are getting excluded.
They might get a neutral tag to for pointing it out to potential trading partners.
But no need to worry here for you because DT is giving people a chance to improve. Or at least I am doing this.
Keep in mind that the issue some years ago was a very clear offense, where many accounts were involved and some more shady stuff.
So, you are safe.
- Is a user trusted based on how long he has been in the forum?: I believe the answer is No. You can be 5yrs+ here and not trusted while a user like EZ0010 with only 5 activities would be trusted much.
An account that has done 100 trades with 1 BTC is more trustworthy than a shitposter with 0 trades.
That's how trust was developed. Some years ago, there was a part where the feedback included a "risked amount" of BTC, to show how much BTC was involved.
More BTC = more trustworthy.
But that "risked amount" was removed in 2019.
I found it very useful but since BTC increased so much in price, it was removed...