that's an awfully broad brush you're painting with. a huge number of hmong that are here today came over as refugees post-pathet lao, which coincides with that age range, but a lot of their children speak it at home as well, even if they're not fluent or well-practiced.
i don't hang out with any of them on a regular basis and we've never really discussed the language, but i couldn't remember if there was a script or if it was just phonetically translated to the roman alphabet; regionally there's a laotian script, a thai script, burmese, and khmer, but it'd make sense that either of the first two could be used to phonetically transcribe the hmong language.
Trust me. The Hmong Americans (born in the U.S.) today in the range of new born to the early 30s doesn't know how to speak, comprehend, or write in Thai/Laos. I'm not sure where you are getting your info from, but we do not communicate the Thai/Laos languages at home. You are either speculating wrong assumptions or someone have been lying to you. I'm assuming you are making your own assumptions only. We grew up speaking english and Hmong, and learning Spanish through school. We the Hmongs are a culture who likes to acclimate to the environment we live in. We take the new step toward the future. We see things of the past as "old fashion". We are open minded people who even though are religious are up to modern beliefs. There's no need for us to learn a language or writing that isn't ours (Thai/Laos) especially when we have no need for it being Americans.
But, around 2003-2004 there have been the new group of Hmong refugees who immigrated to the U.S. Some came here, most went to Cali, and majority went to Minnesota and Wisconsin. They are still learning the English language and are not fluent yet, but are making progress. If you consider them, then they are fluent in Thai and Laos. But if you want to based solely on the Hmongs who were born in the U.S. then I guarantee you that you'll have a difficult time finding one who can communicate the Thai and Laos languages.
The Hmong have no script. The only thing we have in common to the other ethnicity is that we are also Asian. We doesnt share any bond other than similarities in foods to them. Burmese and Khmer are imo even further out than the Thai, Laos, and Viet to us. Like I said we uses the English alphabets very similar to the Spanish writing. Myself along with many other young Hmongs cannot read or write in Hmong. I can speak it fluently but you'd be surprised to know how little I can read and write in Hmong. I mean really I have zero ability to perform reading or writing in Hmong. Spanish is a lot easier for me to read/write than my own language.
You might have already know this but the Hmong doesn't have a country to call as ours own. We live within Thailand, Vietnam, and Laos. The Thai, Viet, and Laos treated us badly in the past that's why we helped the U.S. during the Vietnam War. We have no relationship with any of them or their languages. Thai, Laos, and Viet languages are comparable to the English language. You live there so you learn it to communicate only. It's not a part of us.
You might have not noticed this, but there is a small group of the the Hmong who are currently living in China. Our history dates back to a time where the Hmong used to live in China. China used to be our home. Our real home. The Chinese forced us out so many migrated to Thai, Laos, and Vietnam. Some decides to stay in China. They are Hmong, but they have a unique dialect and accent to their language. Look up the "Miao". They are Hmong whom are left in China.