What is the effect of an ageing population on Italy?

Slower economic growth
When a large part of the population starts to move from the pre-retirement to the postretirement age group, the effect of that is to slow down the growth of the working-age population. Other things equal (that is, assuming given participation rates and productivity growth), this is going to mean lower growth in per capita incomes and lower growth in the total economy.
Strain on government budgets
One way this will arise is through government pension commitments, especially where
governments run defined-benefit systems that are effectively funded on a pay-as-you-go basis. This is borne out by official projections for the US social security pension system, for example, which show that it will soon turn cash-flow negative and eventually will run out of funds.
Effects on financial markets
Population ageing means we are entering a world where, in relative terms, labour will become more scarce and capital more abundant. As a result of that, economic modellers generally predict that real wages will rise and returns to capital will fall. It is interesting to note that this change in relative prices will actually make it harder to prepare for population ageing purely by saving more, because the returns on saving will be lower. This means that what will be needed is some combination of increased saving and greater workforce participation, which is what higher real wages should help to bring about