My mom does this same. She always has to be with me like some sort of hawk. She's afraid that something is going to happen, and I might have one of my severe meltdowns. D: I keep telling her I can do things on my own without her following me. The only time she ever lets me wander a bit is on my own is in the vendor hall at my anime convention. Today though, she almost wanted me to wait for her and my dad to go to Party City, but my dad was like "She can go by herself," and it shut her up. I was able to go into Party City all on my own for a little while.
I know how you feel though. I'm 25, and even though I am very childish, I get treated like a child as well. It can be annoying, but for me it's sometimes a good thing because that's the only way I'll ever learn from anything.
The thing is, parents usually see things that we don't see. Mine often tell me I'm like Temple Grandin for example, but I don't see it because Temple is more autistic than I am. I don't really see my autism as much as they see it. They're only doing what they think is best for me without trying to make it worse for me. I think maybe that's how your mom feels. She thinks she's protecting you from things since she might see you in a different way than you see yourself.
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Currently diagnosed with Autistic Disorder, ADHD, severe anxiety, learning delays and developmental delays.