Reported Speech and another question on Preposition

Hi everyone,

I'm new to this forum and I need your help. I can't figure out the correct way of converting the sentence below into reported speech. Here's my attempt - can someone please confirm if this is right or wrong?

I have been dancing since I was 18.
Reported Speech: She said she had been dancing since she was 18.
(My confusion is on the verb after 'since' - should I keep it as 'was' or change it to 'had been 18'.) What rule applies here?

Secondly, which is correct:
the fourth Thursday of August or
the fourth Thursday in August

I seem to find people using of and in all over the internet and I'm not sure which one is correct.

I look forward to your answers.

Thanks so much.