This is all the more (all the more = much more) worrying(形容词,令人担忧的) given(考虑到,介词) the rapid ageing of Italy’s population.
整个句子的意思是:考虑到意大利人口的迅速老龄化,这件事/这种情况（指整句的上文所说的情况）,而非就越加令人担忧.