I was part of a co-op in Florida before buying a condo. Like a poster has already stated, when you are a shareholder in the co-op more than likely you are considered as one of the owners. If your co-op has a mortgage with a bank they probably require a "reserve" be kept for major repairs. However, if a major improvement, like a resurfacing of roads would be required, or a sudden spike in taxes and the amount of money to repair the roads, or pay the taxes would eat up a large portion of the reserves, then the co-op would have to order an assessment to each shareholder to have the work performed, plus also to keep the reserves at an amount required by the bank.
If there is no mortgage, it may also be that the co-op requires the reserves to have "X" amount of money on hand and if the repairs will eat up a large amount of the reserves , or if you have no reserves, you may more than likely have to pay the assessment, regardless of your age. This is why it is usually a good idea to stay away from co-ops. The upside to a co-op is that those in the co-op generally pay a lesser amount of monthly maintenance fees than a month to month renter.