How to do I say doing some thing for the first time in japanese?
How do I say: "The first time I [did something] was when I was [some age]."
For example, how would I say: "The first time I watched anime was when I was three years old."
would it be watashi wa sansai ni hajimete anime o mimashita. ?

Question

How to do I say doing some thing for the first time in japanese?
How do I say: "The first time I [did something] was when I was [some age]." 
For example, how would I say: "The first time I watched anime was when I was three years old."
would it be watashi wa sansai ni hajimete anime o mimashita. ?

Answer

"The first time I watched anime was when I was three years old."
--> わたしがアニメをはじめて見たのは、(わたしが) 3さいのときでした。
or わたしは3さいのときに、はじめてアニメをみました。

I hope this was helpful.