I don't believe there is a way of telling the age of an orchid on the actual plant. Some orchids are century old. These dates are recorded on paper and the orchid given a clonal name. These were probably divided a few times too, and with excellent horticultural practice they can last centuries.
A large plant with many growths would be old and the number of growths could be a estimate of age, assuming it hasn't been divided before The width of the plant can also be used in estimates. For monopodial plants, the hieght of the plant can also be a estimate. Of course growth rate differs from plant to plant, and with different people and growing conditions. You would have to know how fast the plant species or hybrid grows to give a good estimate. I don't think its a precise estimate of age though. Anything recorded on paper or tag would be more accurate. Some of my tags have the dates the plant was first deflasked.
I say anything big is old I think size is more important than age for most people.
Are there any scientists here doing work on wild plants, estimating age?