Hi and welcome Personally I believe that giving the "age" of people's damaged lungs isn't particularly helpful, it can cause unnecessary alarm - Im 71 and mine is apparently something like 160 and Im still going strong!
I can't answer your question about what 80 means - I don't understand why they were measuring your lung function with a peak flow meter if you have emphysema. Copd (emphysema and/or chronic bronchitis) would normally be tested with a spirometer which measures lung volume rather than flow - a peak flow meter is usually used if you have asthma.
Also it's not worth killing yourself trying to get the best possible result at a DWP test - you need them to know how bad it can be, not the best it can be imo.
If by any chance they did use a spirometer to test you, then likely they gave you the FEV1 result (forced expiratory volume in one second). If so, 80 would be 80%, the percentage of what would be predicted for someone of your age, height and gender if their lungs were normal. 80% would then be borderline mild/moderate stage emphysema.