Is or was "too young for to marry" valid English, now or in the past?

"For to marry"? Was this ever "valid"/common English?

Yes, it was. "For + substantive" still has the meaning of (1) "for the purpose of + substantive" or (2)"in order + to infinitive".

However, the to-infinitive, which has the properties of a gerund, is rarely used with "for" in this sense.

"We can use the iron bar for lifting it." or "We can use the iron bar in order to lift it." (Old version "for to lift it.")

Yale Grammatical Diversity Project English in North America

For to infinitives

"I'm goin' to Louisiana, My true love for to see." - Oh! Susanna, by Stephen Foster

For to infinitives are verbs introduced by for to where Standard English would use to, as in (1) (Henry 1995):

1. I want for to meet them.

In Standard English, it is common to introduce a clause containing an infinitive with for if the verb has a subject, as in (2), where Alex comes between for and to:

2. I want for Alex to meet them.

The non-standard for to construction usually refers to sentences where there is no intervening subject, like (1).

Edit to add:

There is an interesting paper on the history of "for to verb" at http://www.public.asu.edu/~gelderen/forto.htm