In Thai culture in general, senior members must have a high capability and experience to help and support junior level. I understand this point. However, in software engineering which is fast changing, no one expects to know everything.
This is, quite simply, the point you should explain to your boss.
It is always the case that you can find a new, young team member who happens to know about a specific technical area more than other team members.
This is absolutely the case with all technical disciplines.
There is a story I tell people relating to this issue or seniority and technical knowledge which may be useful to you. There was an aircraft crash (Korean airlines). At night the captain became disorientated (a night take off - this can happen) and lost a sense of where the horizon was. He started to direct the plane into a steeper and steeper bank which led, over a period of minutes, to the plane crashing. After the investigation it turned out that the co-pilot, a much younger man, evidently understood the problem and made a half-hearted attempt to alert the captain. But he was not forceful or direct enough, and this is thought to be a result of the culture of the company and his country (possibly similar to Thailand in this respect).
After the accident report was published, heavily criticizing this cultural problem, the airline did react positively and trained all their pilots to, in essence, ignore the rank and cultural aspect of seniority and concentrate only on what they know to be correct, asserting their authority based on knowledge, not on rank or seniority.
Your case is not so safety critical, but the story may be useful to illustrate to your boss the problem with these cultural norms in a technical setting.
Is it very serious for a senior programmer to ask a junior level for help because I do not know a technique or library?
No. The opposite - it is a sign that you understand when you need to accept help and are not too proud or aloof to ask for it from the people you have around you.
The people who join your company, regardless of age or position, are assets. Each one has a unique history and has acquired their own mix of technical and soft skills. It is very, very healthy that a company allows these people to exchange knowledge regardless of age or position.
Does my boss have a hidden message in this?
Only he knows that.
I suspect he is simply showing his own cultural bias, which is understandable.
Then next question, is it American culture to not let a senior ask a junior for help ?
American and western culture in general, especially in technical disciplines, encourage sharing of knowledge without reference to rank.
Having said that, some people (and in some companies) may be reluctant to ask a junior for help or advice. They feel embarrassed or that they are loosing face or, in some cases, they are afraid they will be fired or replaced by the junior.
I think many mangers do not understand that senior technical people are not walking encyclopedias of detail, but are people with more experience of real world problem solving in a commercial environment. Of course they often do have considerable and wider knowledge than a junior person, but that does not mean they should or could have more depth in every subject - it's simply not possible.
In my opinion, age does not matter. Experience is what is important.
Me too. Alas the rest of the world is not as simple. :-)
You must work within and around the cultural limitations of your country, organization and the internal politics of your organization. This is, itself, a difficult set of skills to learn. My experience is that technical people are often blind to these issues, so you are well ahead of the average in this respect.