Conversationally: 

 

How would a Thai person say this? Would the "when" not be used - and it would just be "pom ayoo noi" 

 

For some reason the "mu-arai" in the beginning does not sound right to me??

 

Thanks

TAWN DEK ตอนเด็ก.

Here it is not mu-arai. Just mua (เมื่อ). Mua pom ayoo noi. Or: Mua pom yang dek. Instead of mua you can also use waylaa (เวลา). 

Could use: ตอนเป็นเด็ก or สมัยเป็นเด็ก

I also think it could depend on whether you want to refer to an incident that happened during your youth at a specific point in time, or if you want to refer to the overall time period of youth in general (non-specific point in time). I believe ตอน refers to a specific point in time, while เวลา and สมัย refer to general time periods (non-specific point in time). Somebody please set me straight if I'm confused here. Thanks

 

Edited May 29, 20206 yr by ColeBOzbourne

I think that the reason that many contributors don't bother to comment or perhaps even, read this forum, is because the more one learns the more one realizes that there are many ways to say the same thing and one can not be sure that one is using a common parlance. 
เมื่อผมเป็นเด็กอยู่ตอนนั้น...  ตอนที่ผมเป็นเด็ก... ขณะเด็กผม... เวลาเด็กของผม... ระยะผมเด็กอยู่... etc. all contain words that a Thai would understand, whether the Thai person would suggest their own parlance or not would depend on your relationship to them. I wouldn't normally correct a non native speaker of English saying; Before, I was a child... Then, when I was a child... At that time when I was a child.... etc.  a correction, eg. "in my childhood" would depend on our relationship. 
I think that I could benefit from an up to date Phrase Book now, at least it would give me the confidence to answer questions like this.