Only can be an adverb or an adjective. Furthermore, as an adverb it can modify a verb or an adjective.
I only went there to see Becky.
I took the only piece of candy left.
There was only candy left.
Because of this, the position where only appears is crucial - the speaker/listener will associate it with the closest word.
So all of your sentences mean different things:
I only sell apps to the young person.
You don't do any other action besides selling apps to the young person.
I sell only apps to the young person.
You don't sell anything else but apps to the young person.
I sell apps only to the young person.
You don't sell apps to anyone except the young person.
I sell apps to the young person only.
You only sell apps to one person, and that person is the young person. This isn't too far off from the previous meaning.
Furthermore:
Only I sell apps to the young person
No one else but you sells apps to the young person
I sell apps to the only young person
There is only one young person and you sell apps to him.
This is an interesting questiion. What exactly do they modify? The answer is, of course, if they modify a noun, you need to use the adjective, and if a verb, then the adverb. Now, the first sentence, while common, isn't really correct. [Edit: Fumblefingers' comment makes me think twice about that. Anyway...] Look at these:
If we must kill them, let us do so quickly and cleanly, without excuses.
If we must kill them, let the killing be quick and clean, without excuses.
In your first sentence, these two constructions are getting crossed: it actually refers to the killing. However, it is also more concise than us do so. There are times when informal conversation performs this sort of abbreviation at the expense of absolutely correct grammar. In this case, of course, the most concise version is:
If we must kill them, let it be quick and clean, without excuses.
Clearly, the it refers to the killing. Perhaps the reason that the sentence sticks with the adverb is because the noun killing hasn't been mentioned, so there's a bit of an added thought process to mentally fill it in.
This tendency to abbreviate is probably why you will often see this substitution of an adjective for an adverb, especially in AmE:
If we must kill them, let us kill them quick and clean.
You pretty much have to learn on a case-by-case basis when colloquial English will allow this sort of transgression and when it will not.
Best Answer
Yes.
Not necessarily. Adverbs can also modify adjectives, other adverbs, or in your case, numerals.
No, it still means something like 'four to six people'. If there is no doubt about the exact number, 'approximately' should be dropped.
You could arguably say it requires a verb because every clause requires a verb. But that is true for more word classes than adverbs ...