the Constitution

I'm reading the constitution of the U.S.

There appears '...who shall not have attained to the Age of thirty Years...'

-For 'attain to', it seems that 'to' is unnecessary, but it has. Is it because the Constitution was written in the past and the usage is still preserved in the Constitution to this day?

-Why 'Age' and 'Years' have a capital at each's head? Besides those, there are many words with a capital on the document.

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Below is about 'The Senate'
'Immediately after they shall be assembled in Consequence of the first Election they shall be divided as equally as may be into three Classes'

-For 'as may be', is a subject intentionally ommited between 'as' and 'may'? Or is it appropriate as it is?

<<For 'attain to', it seems that 'to' is unnecessary, but it has. Is it because the Constitution was written in the past and the usage is still preserved in the Constitution to this day?>>

Yep. In modern English usage it would just be "attained the age" (although this construction itself would still be very formal).

<<Why 'Age' and 'Years' have a capital at each's head? Besides those, there are many words with a capital on the document.>>

In the traditional English legal style, all nouns (or at least the most important nouns) started with capitals.

"-For 'as may be', is a subject intentionally ommited between 'as' and 'may'? Or is it appropriate as it is?"

Nowadays, we'd probably just say "as equally as can be" or "as equally as possible". I suppose you could also write "as equally as they can be", so you could say that a subject was left out here.

This seems to be an intransitive use of the word 'attain' if what
Dictionary.com says below is any indication.

v. intr.

To succeed in a directed effort, process, or progression: attained to high office; eventually attained to wisdom.

This is typical of legal language even though the spoken language would use transitive forms without the preposition 'to'.