As noted, it is varied; so varied I'm not going to attempt to tackle it across the board. I've worked for a variety of employers with tactical teams of one variety or another. The minimum requirement among them was 3 years on duty demonstrating consistent performance under pressure, the earned respect of everyone from peers to management to trainers, and above average knowledge, skill, and ability across the spectrum of skill sets encountered in the field. And the physical and firearms benchmarks, lets not forget those. Realistically, the guys I know on tactical teams are typically in their mid-30s to mid-40s, with military and law enforcement backgrounds dating back to their high school and/or college graduation. The better outfits are extremely challenging to earn a spot on, it is that competitive.
Most departments and agencies do not have tactical units. Of course, if your definition is broad enough to include a handful of guys with some extra training and equipment, there seem to be lots of units. I adhere to a narrow definition; think FBI HRT, ICE SRT, USMS SOG, NYPD ESU, LAPD SWAT, etc. What do they have in common? They are top tier outfits, funded, trained and equipped far in excess of "the norm," capable of handling far in excess of "the norm." Most "tactical teams" these days seem to be "low risk" units with a couple of grand per member in equipment and hopefully, maybe, a dedicated training program to bring things together. That. Does. Not. A. Tactical. Unit. Make. Don't get me wrong, those guys are in the trenches, those trenches are dangerous, and they should be well prepared for it. They are a different thing, that's all.
Too varied to say. There are full timers and part timers, with the majority being part timers.
I'm not a member of a tactical unit. I train and have trained with a variety of tactical units members, though; federal, state, and local. They are extremely motivated individuals who have worked hard to get and stay where they are, hoping to get all they can from it before they have to hang up their gear as age and the injuries mount up.
Now, all that said... Get hired, put three or so years behind you, volunteer for a "low risk" outfit (warrants, drugs, the stuff you wouldn't need a top tier unit for), and spend a few more years proving yourself there. Remember, no matter how fit you are or well you shoot, if you don't earn the respect of the right folks it won't matter, assuming the outfit you are interested in is worth competing to be on; because competing is what it is and should be to ensure the unit gets the best available.
Meriam Webster's: Main Entry: ci·vil·ian Pronunciation: \sə-ˈvil-yən also -ˈvi-yən\, Function: noun, Date: 14th century, 1: a specialist in Roman or modern civil law, 2 a: one not on active duty in the armed services or not on a police or firefighting force b: outsider 1, — civilian adjective