what is the age of an old piano according to serial number?

Production began in 1851 and ceased in 1976.

http://www.pianoatlas.com/howold.html
The Pierce Piano Atlas provides a fantastic source of information about manufacturers and the dates pianos were built. Our web site, PianoWorld.com is dedicated to providing information about pianos. We often refer to this book to find the answers our visitors are looking for. Although there are other books that provide this type of information, none seem as complete as the Pierce Atlas.

If there is really an inscription there calling the manufacturer a "Hoffabrikant" ("Manufacturer of the royal court"), the piano was probably made before the revolution of 1918.

The price is very hard to estimate because on the one hand, the sound is not going to be up to modern standards even if the piano is in good condition for its age. On the other hand, it might be a collectors' item, which would push the price up. I'd talk to a professional, perhaps one at an auction house.

If it is any help. I have a W Biese piano no 14326, which I know was bought secondhand inDeptford in 1920 for £100. It had been black lacquered, but was originally mahogany with candle holders which were removed before it was laquered. Sounds nice, but not as loud as a modern one, - perhaps a good thing!
Darlingirl.

Thomas - there's a Norwegian lady asking for help with a Biese piano; do you think that you, or anybody else here, could help her? Thank you:
http://able2know.org/topic/174176-1#post-4658011

Try to mail your questions to Biese's e-mail address, [email protected] (Thomas (c)). This method helped to recognize the piano with serial number 5418 (from https://able2know.org/topic/174176-1#post-4658011)