That's not an assumption, but an observation. The universe (meaning all the space we can in principle geometrically travel to) is of finite age. We can observe the development of the universe from an extremely primordial state (we can see directly that it was a radically different place) and measure accurately (by several methods) how long ago that was.
The only way that could be an observation would be if you were there fifty gazillion years ago and SAW the universe being created. Anything else is in fact a collection of assumptions, along with a few observations of phenomena which can be observed, measured, and INTERPRETED, in accordance WITH the assumptions.
Other than that, I notice that every other sentence on your FR page begins with 'I' or 'My'. That's the basic characteristic of an egomaniac, not somebody interested in discovering the laws of the universe or whose judgement on what was going on fifty gazillion years ago I would view as meaningful.