This question has 2 parts.
Why is it that ああ has so many different kanji 嗚呼; 於乎; 於戯; 嗟乎; 嗟夫; 吁; 嗟; 噫; 鳴呼 (source)
and is the average japanese (16 yr old and above) able to recognize them all?
These seem to mostly be cases of 義訓【ぎくん】, where it the kanji are only used for their meaning. The most common of the kanji compounds (鳴呼) for example is comprised of kanji which carry the meanings of the sound of crying and "to call out."
As for why there are so many, I would say that it is because there are so many ways in which one can cry out. Judging from the meaning of the kanji, 於戯 for example would likely mean ああ either during or after playing sports, while 嗟夫 seems to have a slightly racier meaning. However, in my experience, very few of these are used commonly.
At the very least, the average 16-year-old will very likely know at least 鳴呼, as it is used in manga and novels aimed at teens not infrequently. It seems unlikely that they would know them all, however, as many of those kanji are not generally used.
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