Hei Lu! :D
While your essay has such a wonderful structure and don't get me started on your very "I-had-to-read-your-essay-with-the-dictionary-next-to-me" vocabulary, I don't fully like it. I think the arguments you made in the second paragraph are far stronger than the ones you described in the second. I think it is not ok to ignore a reasonable concern by just mentioning "the children can benefit from it" while it isn't even true on the long run. That's just a thought though.
Others are concerned about the costs of supporting this huge amount of aged groups in a pension system may be a financial problem for government, especially when an economic recession occurs.
Furthermore , when financial crisis comes up (/when stricken by a financial crisis), the economy will not easily resuscitate with its domestic consumption reliant(/relying) on an ageing population, which is more worried about its health and security in its dotage. -- Not all old people are senile, but ok. XD Just hope the one correcting the essay won't be old.
All in all, the fact that throughout the world ageing population is an unchangeable trend
. It has both upsides and downsides. -- You were missing the predicate for the main clause, so I put those two sentences together. However, this sentence is incorrect. Societies where the elderly overwhelm the providing faction of the country do not include major parts of Africa and South America, but quite a lot of developed countries such as Germany, Russia or Japan.