Size of plants isn't always a good indication of age, and that's especially true of plants grown indoors. Poor cultural conditions suppress growth to the point where the plant can remain in something of a limbo, no growth, caused by the plant being only able to produce as much energy during photosynthesis as it uses during respiration. Plants can only grow when they are able to create more food/energy than they burn, and the greater the surplus of food/energy created, the more rapid the growth rate.
The key factors affecting growth ate light levels, appropriate fertility, soil air:water relationships, and watering practices, with other cultural influences also playing perhaps a lesser but nonetheless an important part.
Too, plants don't age the same way as humans. Plants age ontogenetically while humans age chronologically. For more re this, use search words plants "ontogenetic" aging. If you DO apply a chronological age to your plant, it would have 2 ages. The first would be a measure of how long it's been since the plant first grew roots and a stem, the other being how long you've had it, and we have no way of knowing either.
Has your plant been doing well, or do you have any concerns re growth, vitality, eye appeal?
Al
