The reasons for the Trojan war are:
1. Paris took Helen to Troy.
2. Helen was Menelaus' wife.
3. Menelaus loved his wife dearly.
Helen of Troy was played by Diane Kruger in the movie Troy.
The love interest of Brad Pitt (Achilles) in the movie was Briseis played by Rose Byrne.
Helen of Troy was the daughter of Tyndareus and Leda, the king and queen of Sparta. Zeus appeared to Helen's mother in form of a swan, seducing her. Helen of Troy has two brothers and one sister: Castor and Polydeuces, and Clytemnestra. Helen then married Menelaus, the brother of Clytemnestra's husband, Aganemnon, and then later ran off with Paris of Troy.
because she cheated on her husband with the prince of troy
At the end of the Trojan War, Helen of Troy committs suicide because she feels like it is her fault that so many people died and all the guilt is on her.
The event of Paris kidnapping Helen of Sparta (or Troy, whichever you prefer) is part of a supposedly fictional story, so therefore there is no evidence. Not only that, but if it did happen, it was thousands of years ago, so if there was any evidence, it's probably gone by now.
Their exact ages are unknown, but I'd estimate Helen to be about 17 or 18, and Paris to be about 19 or 20.