I do not actually know how many years this place has been in operation, but I am confident it has been for many years and is one of the more established places in town. It is right alongside the Democracy Monument and is about a 100 BHT cab fare out of downtown.
Without exception, this is one of the best authentic and best quality restaurants in Bangkok and the menu is really extensive and includes so many outstanding dishes, you will need some time to work through
https://www.tripadvisor.co.nz/ShowUserReviews-g293916-d2038694-r143652171-Methavalai_Sorndaeng-Bangkok.html#