Take it to a firearms dealer or appraiser. There are also plenty of books on antique firearms at your local library.
Where did you find the serial number?
SN's were not required at that time. If it has one, look on the butt or frame.
Research of old gun catalogs and books.
The lower the serial number the younger the pistol is
The PY in the middle of your serial number dates your pistol to 1982.
Caliber should be on the barrel. SN's were not required until 1968
You will have to call Colt with the serial number to find out.
Impossible to answer without the serial number.
No way to answer without the serial number.
The year of the Colt 25 pistol with the serial number 255465 varies greatly depending on the actual model of the pistol. That serial number in the model 1908 might date back to the early 1900s. That number in later models can be from the 1950s.
No serial number = no answer.
In order to tell how old your Colt 25 semi auto pistol is, you will need to offer the serial number. This number gives you an idea of when the pistol was manufactured.