Source: The Beginnings of Rome, T.J.Cornell
The people of Rome was divided into centuries, according with their wealth, in five classes, making the Comitia Centuriata.
According with Livy and Dionysius of Halicarnassos, the organization was so (around 211 BC):
(class / property rating / number of centuries)
Class I 100.000 80
Class II 75.000 20
Class III 50.000 20
Class IV 25.000 20
Class V 11-12.500 30
Supernumerary: equites (18, normally placed in 1st class); engineers (2, 2nd class); musicians (2, 4th class); proletarii (1, no class).
The classes above are further sub-divided in seniores and iuniores, half each. The iuniores are the ones with age 17-45, and the seniores, 46-60. Because of the mortality, a centuria senior will have much less people, perhaps a third of a centuria iunior.
Method of assesment: "membership of the classes depended on an assesment of the value of a familiy's estate; the paterfamilias and all free-born males within his potestas were then assigned to the appropriate class". This assesment of value is measured in "as"/"asses", a pound of bronze.
So, if you is (that is, was) a roman citizen, and the Censores evaluated your fortune as being, say, 60.000 asses, then they will put you in the III class. If you is poor, without even a slave, then you is in the last centuria, one of the proletarii.