Hi Dej,
Here I am, just saw this thread (obviously)
I've never heard you sing in "LIVE", so it'll be very hard to know your voice tone.
In mid 70s, I've had a good chance to take voice class when I was taking Music as Major at Glendale City College.
I've learned with Professor Paul Mayo who tought us about how to sing (or get the best out of what we have to sing), and I passed his final test of his class too. And that meant I had received something from him that made me a better singer than I was before. Prof. Mayo was a tenor singer and surprisingly not a big man (about 5 foot 9 inches and 170 lb from my observation) but his voice was so strong and steady, I couldn't believe when I first heard him sang!
To make long story short, I believe you still can learn to make the best of what you got. I'm sure there are many books about "SINGING" in Thai that you can try and they shouldn't do any harm to you. To me personally and you may know already it is about control your breathing at the right time/note is the most important thing to start with.
For example there is an old folk singer named "Joan Baez" that you can learn about breathing from, but first of all, how many octaves that you can sing from your lowest to your highest pitch? I'm sure you know that by using the piano keys ( or keyboard) to test it out. So you will know what you've naturally got right there (may be 2 octave or even 3 or more octave) and also how long you can sing in 1 note (in seconds if not minute) etc.....
Those kinds of stuff will let you realize of the arsenals that you already have and will be able to working around them to your advantages.
You are certainly not too old to improve your breathing and play around with your vocal cord.
Just read those books and you'll see what I mean.
I truly believe it's mainly not the quality of your voice that will execute the missions, it's the intention, determination, and sincerity to convey the message of the song to the audience.
As we know there are so many singers of different voices that captured the hearts of people all over the world and not necessarily in English or any particular language. Clodine Longet, Astrude Gilberto are at least two of those that I'd encourage you to listen to, they seemed to fit in your voiced catagory and you might as well listen to Sade also (check them out in YouTube I supposed)
Well, it's getting longer again, sorry for the rambling. Check it out by yourself and let me know if it helps OK?
http://www.youtube.com/watch?v=0UEzXdxvuxc
http://www.youtube.com/watch?v=G9U6URQSF6U
http://www.youtube.com/watch?v=QiTA2jaXOLE
And bonus here's "Diana Krall's Cry me a river"
http://www.youtube.com/watch?v=S4hPii_RVHE