Should the age be 21 for firearm purchases in the U.S. ?

Recently I've been hearing arguments about the age of firearm purchases being raised to 21. The argument I heard was in support of this, but I disagree.

In the U.S., handgun purchases are restricted to 21 and older. Long-gun purchases can be made by anyone 18 years of age that can pass the required federal background check.

I'm against raising the age to 21. Why? Are young adults not supposed to defend themselves? 

Imagine if this were the law for a moment...

Imagine a young man or woman living alone, are they supposed to be unable to defend themselves? 

What about a 20 year old young woman with a child and a restraining order against her abusive ex-husband or ex-boyfriend? She's not supposed to be able to defend herself?

So you can move out and buy a house, a car, have a dog, a child, and not be able to defend any of those things? 

We can vote for our leaders at 18, but can't enjoy the freedoms we have as people?

We can serve in a military at 18 without parental consent yet we would not be able to purchase firearms privately?

I disagree on this, it is fundamentally wrong. We as people have the right - simply by being human - to be able to defend ourselves. Allowing artificial limits on these rights would empower the thought that they are not ours for simply being human, but are given and taken by the state - which I deeply disagree with.

What are your thoughts?