If you are multilingual and make use of more than one language regularly, you are in luck! There is a growing body of evidence that confirms the benefits of being bilingual; chiefly the positive effect it seems to have on cognitive performance. In recent times, we are also seeing more evidence  about the protective effects bilingualism seems to have against dementia and cognitive aging. On average, bilinguals seem to develop dementia five years later than monolinguals.
Bilingual people are better at carrying out tasks that require focusing on one piece of information without being distracted by others. Years of managing interference between two languages have made the bilingual brain an expert at executive control.
A recent research  of brain activity directed by Dr. Ana Inés Ansaldo gives us more insight into how this happens. The study showed how monolinguals need to involve and connect five different brain areas to solve the same task that bilinguals can do with fewer and more clustered regions. The bilingual brain is more efficient whereas monolinguals seem to consume more neurofunctional resources. This could be making them more vulnerable to cognitive aging.
In a review of nine recent studies on this topic , Amy L Atkinson indicates that the protective effect against dementia doesn’t seem to occur when the second language was learned later in life or when it is not used frequently enough. However, different studies use different definitions of bilingualism, which makes it hard to draw a conclusion about what type of bilinguals benefit from these protective effects against cognitive decline.
I would also like to note, that most studies that I have talked about so far, make use of visual tasks to assess bilingual and monolingual cognitive performance. A different study measuring auditory attention  showed that the effects of bilingualism in the auditory domain are not confined to childhood bilinguals, extending to early and late bilinguals.
It seems that both early and late acquisition of a second language is linked to enhanced cognitive performance as well as cognitive flexibility during multitasking. If you are monolingual, maybe this is the final push you needed to start learning a second language!