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Following on from the question "Using Genitive & Infinitive To Describe Characteristics"; Joonas (26/6/19): "adulescentis est maiores natu revereri." = "It is of a young person to respect his/ her elders/ ancestors.

What is the purpose of "natu" given that "maiores" means "elders/ ancestors", without any help?

At fist sight "natu" = "by birth"; but, that does not fit here.

Looking around the net: "natu salu" = "health leaders". Think salu should be "salutes"?

"My Memory" site: "a Mileto autem mittens Ephesum vocavit maiores natu ecclesia." = "And from Miletus he sent to Ephesus, and called the elders from the church."

"nec corripuit eum pater suus aliquando dicens quare hoc fecisti erat autem et ipse pulcher valde, secundus natu post Absalo (Absolem)." = "And his father had not displeased him, at any time, saying: "Why did you do this?" And he, himself, was a lovely man; and his mother bore him after Absolom."

Where is the ref. to "mother" here? Is it "secundus"; then "secunda"--the-secondary-one; or, the-following-one; wife being seen as a chattel (assumption)?
Perhaps "secunda natu" = his wife, by (gave) birth...? (My confidence is low.)

"venit Moses et convocatis maioribus natu populi exposuit omnes sermones quos mandaverat Dominus." = "And Moses came, with the Elders of the people, having been called and laid before them all the words which the Lord had commanded."

Any thoughts on "natu"?

The noun natus you encountered can mean "birth" or "age" or similar things as the dictionary entry quoted in Rafael's answer points out.
The ablative natu is often an ablative of respect, explaining in which sense someone is maior than someone else:
maior natu = "greater with respect to age" = "of greater age" = "older".

The word maior(es) can be used on its own, but adding natu makes it clearer that the reference is to age or some other kind of seniority.

The same reasoning goes with secundus natu: it is "second with respect to birth", meaning a child born after another one.
The Latin sentence you mention phrases the birth order of the siblings using natu, English uses being borne by the mother.
It is the same message, but delivered in different words.

I should also point out that the participle natus means literally "born", but is far more flexible than the English counterpart.
For example, viginti annos natus is "twenty years old".
The conflation of birth and age is very similar in the participle natus and the (defective) noun natus.

According to L&S, this is idiomatic. It is a defective male noun, used only in abl. sing., meaning birth, age, years, hence maiores natu—older in age, etc.