Translation:I saw the Hagia Sophia for the first time when I was fifteen.
No, that sounds very clumsy. Try and always say "ilk kez + time expression".
Ayasofya'yı ilk kez 2005'te gördüm.
Ayasofya'yı ilk kez üniversitede(yken) gördüm.
Ayasofya'yı ilk kez evlenince gördüm.
and so on.
Alternatively, you can "relative clause"-ify the "gördüm"-part of the sentence:
Ayasofya'yı ilk kez gördüğümde on beş yaşındaydım. = When I first saw Hagia Sophia, I was 15 years old.
And by the way the "kez" is omittable in all of these sentences.
Is Ben a person? (Like Ben Affleck?) Otherwise, it's öğrendim*
Both are correct. They just stress different things:
Ben onun dün oraya niye gelmediğini Alan'dan öğrendim.
→ It's from Alan (and not from Ektoras) that I learnt why s/he didn't show up there yesterday.
Here's how I say this sentence: http://vocaroo.com/i/s0IbIM32Mrem
Ben Alan'dan onun dün oraya niye gelmediğini öğrendim.
→ What I learnt from Alan was (not what his or her name was, but rather) why s/he didn't show up there yesterday.
Here's how I say this sentence: http://vocaroo.com/i/s1sfexN5h3ze
P.S. I've assumed in both sentences that I learnt from Alan why some specific person (other than Alan himself) didn't show up there yesterday.
Kez and kere have same meaning and We use kere and kez,when we want to say repeated action. For example "Oraya 2 kez/kere gittim." (I went there twice) "Zaman" and "Vakit" are different from kere and kez and "zaman/vakit" have same meaning and we use generally them simple. "Ne zaman/vakit geleceksin?" (When will you come" "Zaman/Vakit geldi mi?" (Did The time come)