Although it might appear to be an important thing to those just getting into bonsai, age is vastly unimportant for bonsai--unless you have a tree that exceeds 100 years or more--which is common with collected trees, not so much with nursery grown stock like your Carmona or Fukien Tea. "Real" age can alter how you care for a given tree, as older specimens may not appreciate aggressive "feeding" and root care for instance. Younger trees can be a bit more resilient.
There is no completely accurate way to judge age of a bonsai, unless as pointed out, you can actually see the growth rings inside the trunk, or have records of care for it from previous owners.
With tropical trees, counting rings really isn't accurate either, as tropical species do not experience yearly season-driven growth cycles as temperate trees species do. Tropical species mostly grow according to availability of water--monsoons, etc. They can have several growth spurts during a given year, depending on that. Nursery grown trees don't even have to rely on natural conditions.
Your tree is most likely sourced from a grower who has a greenhouse and propagates thousands of seedlings a year under steady conditions of light, watering and fertilization. It's probably a safe to estimate your tree is likely about 5-10 years old--with 10 being a bit on the higher side of possibility.
All this Doesn't make any difference, really. The bottom line question for ANY tree, is "does it LOOK old."