Most babies start using a sippy cup once they reach 6 months of age and then continue using the sippy cup until around 2 years old.
Some 2 year olds are ready to drink from a regular cup and give up a sippy cup but some 2 year olds may not be ready yet.
For example in the car or house when they are prone to spilling drinks the sippy cup can be a handy thing as it prevents spills.
My kids continued using a sippy cup until after 8 years old in the car or when they were at the computer to prevent spillage.
Even my current 8 year old son uses a sippy cup when he's on the computer as he's spilled his juice and sometimes milk on his computer keyboard.
If he knocks the sippy cup over he doesn't spill it.
I think sippy cups are a good thing even for adults or anyone of any age.
I sometimes use one myself when I'm working from home and need a drink and don't want to spill the drink on my computer.
They are considered just for toddlers and babies but there's nothing wrong with anyone using a sippy cup.