From a little bit of research, I found that anyone can be any node other than a guard node. Why is that so? On some forums they say stable uptime for a few weeks is required, but is there a minimum requirement?
In the Tor article, The lifecycle of a new relay We see that a relay node could become eligible as soon as 8 days after it has been created but in reality it needs to show a longer term history of good uptime and available bandwidth.
Here is the spec sheet. There appears to be at least two flags that must be set to become a guard:
"guard-wfu" -- WFU (in seconds) required for a relay to be marked as guard. "guard-tk" -- Weighted Time Known (in seconds) required for a relay to be marked as guard.
However nowhere that I can see are the actual metrics discussed for achieving these two flags.