In my workshops I often start with a candle burning that I lit before the workshop began. I ask: "If the candle is burning at a rate of one inch per hour how long has the candle been burning?" The students will ask; "When did you light it? How long was it when you lit it?" I don't answer them but ask; "Has it been burning for several days?" Of course they realize that it could not have been burning that long. This activity is an illustration of an upper limit clock. Although we don't know how long it has been burning we can conclude it has not been burning for several days. A candle that LONG would bend over of its own weight, besides they don't' make them that long. So it is limited to a maximum time of burning of hours and not days. There are many processes in the Universe that qualify as upper limit clocks and a vast majority of them indicate the Earth is young. Here are just a few examples; accumulation of salts in the ocean, helium the atmosphere, erosion of the continents, etc. Only the radioactive clocks give ages of million or billions of years.
Recommended reading:
The Young Earth by John Morris
The Mythology of Modern Dating Methods
Starlight and Time