There are actually six stages of Evolution necessary for what we see in today's world (cosmic, chemical, stellar/planetary, organic, macro, and micro). Only one stage, Micro-evolution, has been observed, and the rest are merely assumed. Many of the assumptions have actually been shown to be unreasonable (for instance, the lack of transitional fossils has severely affected the theory of macro-evolution, and the observation of retrograde motion, "voids" and "clumps" have proven to be a great setback to the theory of cosmic-evolution, starting with the Big Bang).These six phases are in detail:(1) Cosmic Evolution (the origin of space, time, matter and energy from nothing); (2) Chemical Evolution (the development of the higher elements from hydrogen); (3) Stellar and Planetary Evolution (the origin of stars and planets); (4) Organic Evolution (the origin of organic life from a rock); (5) Macro Evolution (the origin of major kinds); and (6) Micro Evolution (the variation within the kinds). Only the sixth phase has been observed and documented. The first five are merely assumed. Interestingly, the fourth assumption is the old doctrine of Spontaneous Generation - organic life developing from inorganic matter (a rock).
There are actually Limiting Factors that limit the possible age of the Earth (like moon-drift, magnetic-field decay, Earth's slowing rotation, ocean-floor sediment build-up, chemical influx into the oceans, etc.).