Determining the age of a piano
Determining the age of a piano is a relatively easy process. Unfortunately, I have not been able to find a complete database of this information on the internet. If you can find the serial number of your piano and know the maker, the year of manufacture can usually be found in the Pierce Piano Atlas. Most music stores and technicians have a copy of this book, or you can check your local library. There is also a website that can provide this information for you for a small fee (some links here to specific piano makers that provide this service without a fee). Click here to access this page.

If you need help finding the serial number of your upright piano look here.
If you need help finding the serial number of you grand piano look here.

Determining the value of a piano
Determining the value of a piano is a different matter. There are so many variables in determining piano value that a hands on check of all of them by a technician or experienced piano dealer is necessary in most cases. Expect to pay a fee for this service. It is a must to look at the piano finish, style, age and general exterior condition, condition of keys, action, bridges, strings, soundboard, and tuning pin tightness. The marketability of a particular instrument and demand in your area are also factors. Most technicians cannot determine this even if you give a detailed description over the phone. Sorry, but I cannot reply to requests to assess the value of your piano.

Information on how to become a piano technician
There is an organization called the Piano Technician's Guild that you might look to for more information on becoming a piano technician. There may be a chapter near to where you live, and if so you should visit when they hold a local meeting and meet some of the members. The Guild offers classes at area, regional, state and national conventions that are helpful resources in learning the piano technician trade. Search you local library or the internet for books relating to piano tuning. The Guild puts out a journal monthly that is a great source of information.  The Piano Technician's Guild website is here.

There are two other resources that you can also consider.  There are some colleges and universities that offer 2 year certificate programs in piano technology.  There are also some correspondence courses you can take, such as from the Randy Potter school of piano technology, found here on the internet.

The very best way is to find a reputable piano store that does piano restoration or a dedicated piano rebuilding shop.  If you can obtain employment and apprentice under an experienced technician here, you will be well on your way.  Learning piano technology is a hands-on experience and requires a great deal of time, patience, and practice.

One more interesting question

I have an old upright piano with very dirty ivory keys. How can I clean these without damaging them?

I have seen many of these old pianos with varying degrees of ivory key quality. You did not mention if there were chipped or missing tops, etc, so I will assume that they are in good condition, and that you would like to keep the set intact. If there are only a few chipped keys, there is now a repair that can me made to the chip much like a dentist bonds teeth. Many times, ivories are so yellow or chipped or worn that it is best to just replace the key tops with the plastic type used on more modern keyboards. I actually prefer the feel of the plastic to the ivory, but that is a personal preference, and many prefer the ivory. To me it feels sticky.

Ivory is porous, and because of that should only be cleaned carefully with a mild soap (as dish soap) and water. Use a soft cotton cloth, wet with the soapy water, wring well, and wipe the keys clean. Remember that a lot of the dirt comes from the sides of the keys, so look at the black keys on the sides for grimy areas, and clean off the grime and also the sides of the whites. Your cleaning will last a lot longer if you get that off. Be careful as you are cleaning not to pull on the key top in an upward direction, as that can cause loose key tops to pop off. It you this happens or if you find some loose ones, go ahead and pop them off carefully running a sharp knife under the key top. Use super glue to glue them back. Place dots of the glue on the underside of the key top, and carefully align and set in place. You have only a short one shot at this, so you have to do it right the first time. Never use any slow drying wet white or yellow glues to replace a key top, as the ivory will curl, and the results will be poor. Incidentally, ivory likes light so keep the fallboard lid open. It helps to keep them from turning yellow.