Well. The" tú" it is optional. I ask you ¿Cuántos años tienes? ¿ Tú ,cuántos años tienes? ¿ Cuántos años tienes tú? The last two sentences are redundant or emphatic. Y tú ¿ Cuåntos años tienes?
Can I say "¿Cuántos años tú tienes?" as well or is it more natural (or perhaps a rule) to use the pronoun ("tú" in this case) at the end of the sentence?
It's not a rule, but more a matter of accepted style. Remember that the pronoun is even optional, so "Cuántos años tienes?" is perfectly fine.
Using the pronoun would indicate emphasis or contrast, like saying, "Ella tiene ocho años, y ¿cuántos años tienes tú?"
I have been to numerous spanish speaking countries nobody uses the tu at the end
"What is your age?" is as colloquial as "How old are you?" A literal translation doesn't work so we have to guess the idiom Duolingo might choose. Now I prefer to guess, catch and deduce the Spanish meaning rather than be too concerned about Duo's opinion.
Should this be: Cuantos anos tienes tus. Translation: How old are you both? You two? Or you all? All of you?
Tienes is always a singular second person, and never refers to "you both" or "you two" or anything plural.
If you want to pluralize the sentence, it would be "Cuántos años tienen ustedes?" You change the verb to plural and the "you" pronoun to the plural form.
Tus is only ever a plural possessive pronoun, such as tus libros (your books).