how would i know how old is my model?

The following website (everymac.com) has every mac/portable model:

https://everymac.com/systems/apple/index-apple-specs-applespec.html

You can click on the general model of computer you have, then you can select more specifically the model you have.

If you press on the arrow for your model, a menu will pop down. You can find the introduction date and discontinued date

in there.

Also on every mac you can lookup any of the following:

• Order Number (Example: MA970B/A, MB467LL/A, or MD093E/A)
• Model Number (Example: A1278 or M8493)
• EMC Number (Example: 2254 or 380B)
• Model Identifier (Example: MacBook5,1)
• Serial Number (Example: W88401231AX or C02CG123DC79)
• Intel processor number (Example: P8600 or 4980HQ)

Go to this path:

https://everymac.com/ultimate-mac-lookup/

I hope this helps!