Age-wise, yeah, that's pretty ridiculous. Saying someone has to stay in their room or not watch TV or get internet or be in bed by a certain time, that's pretty silly to say to another adult, regardless of whether or not they're your child. But "getting grounded" as a general concept can happen at any age; I think it's really mostly related to where you're living and/or financial arrangements, more than how old you are. (I'm about to say "you" a lot, I don't actually mean you, just a person in general) If you're at an hourly job and get unprofessional with a customer, you can get temporarily cut hours or wages or even demotion until they feel you've proved your worth. If you live in an apartment complex or rent a home or even have a mortgage, if they catch wind that certain living standards haven't been met, they can give notice of inspection to make sure you fix the place up before they send you an eviction notice. Hell, if my mom moves in with me in her golden years, I'm gonna let her know very clearly that she has to follow my rules while she's under my roof just like I had to while under hers.
If you can support yourself domestically and financially with no one to answer to, that's the only real independence possible (and even then, you still have to pay taxes and follow health codes and building codes to keep the government appeased).
When I was 20 I was in the Navy, living on the ship and receiving monthly pay for fulfilling my duties. If a person fails to follow their rules, they go to see the Captain (literally referred to as "the Old Man") for punishment, and most of the time they're put on "restriction" (essentially grounded) for up to 4 months. They can't ever leave the ship (the house), have to muster multiple times a day (to make sure they haven't snuck out), have to be in bed a short while after last muster (a curfew), and of course they lose an amount of their pay (or cut their allowance) for the infringement. Screw up again during that time, and the restriction can be started all over again, given even more time, or just kicked out of the Navy entirely. Most Captains follow a third strike standard, as an unwritten rule, but if before that third strike they deem a person unfixable or the infringement especially egregious, they'll usually go straight to Bad Conduct Discharge.