Today, in some countries, young adults account for a comparatively large proportion of the population compared with older people. I believe the upsides of this situation far outweigh its downsides because young people are the main drivers of economic growth.
One disadvantage of having large numbers of young people may be that it can lead to high unemployment among them. Most older people already have their careers figured out. They either have stable corporate jobs or run their own businesses. In comparison, many young adults, especially those who just graduated from university, are still looking for jobs. When there are too many of them competing for the same positions, their chances of securing one can be very low. However, I do not think this is a major disadvantage for young people as long as they make the leap to self-employment.
On the positive side, young people are the driving force of an economy. This is because they tend to be more productive and creative than older people and thus are more likely to bring change and innovation to their countries. Take for example the highly innovative trillion-dollar U. S. tech industry. Most employees in this industry are only in their late 20s. I think this creates a great advantage for a country because a strong economy offers people high standards of living.
In conclusion, although large numbers of young adults can mean high jobless rates among them, I believe this disadvantage is greatly outweighed by the advantage that they can drive an economy to grow.