It seems to me that theres no fundamental law of biology which
excludes the possibility of a multicellular organism living a very,
very long time. There are trees which are thousands of years old and I
can imagine vegetative fungi living for tens of thousands of years or
I'd say there are 2 main reasons this doesn't occur in most animals:
First, there is a tendency for all complex things to fall apart unless
they're repaired at a rate equal to or greater to the rate they decay.
Evolution has selected for animals that avoid this decay at least
untill reproductive age but as time goes on it becomes less and less
advantageous as the individuals chances of dying due to disease or
predatation get higher and higher. Another way of saying it is that
entropy will win unless its positively selected against since fighting
entropy uses up valuble resources for the long term that could be used
for the short term advantage.
Second, it would be disadvantageous for the long term survival of
the species. To avoid a population explosion a very-longed-lived
species would have to compensate with a very low reproductive rate.
This in itself would be bad because they'd be more succeptable to
enviromental catastropheses but theres more: sex and reproduction
allow for new combinations of genes which provide the raw material for
evolution. So a long lived species would evolve slowly and have trouble
adapting to new enviroments.
I guess theres a balance between having a rapid generation time, fast
evolution and by neccesity, knocking off the oldsters to make room for
the young fertile ones, and having a slower generation time which
allows the oldsters to care for and 'teach' the younger generation