The say to start at 25. the rationale is that everything is fully developed.
There really is no "exact" number. you dont magically become fully developed at 25 but it seems to be the most agreed on.
I was waiting until I was 25 and tore my chest. it killed my drive for the gym so I had no reason to start. I am in my 30s now and decided to run a cycle after becoming a TRT patient.
Other concerns are fertility.
I am a believer of ARB to counteract some side effects of steroids as it is beneficial for most., vitamins, fish oil. Things that you would take to improve your health even if you were NATURAL.
Learning to train is a big one as well. Different body parts technique, injury prevention. I am always in fear of injuring myself and not being able to lift.
Also make sure you have plenty of funds for "oh shit"
But since this is what you are asking
Basic is
Free and total test.
estradiol
Complete blood count
Comprehensive metabolic panel
Lipids
As time goes on, adding more substances. it might be good to add in more labs
Thyroid panel
prolactin
always have an Aromatase inhibitor or nolvadex on hand for "oh shit" moments.