If you want to slow cognitive decline or improve mental flexibility, then learning a second language, even if it's started later in life, does make sense, but expecting it to improve intelligence or memory overall is probably a beautiful but unrealistic fantasy.
'Second language' is a technical term meaning a foreign language learnt after early childhood. For example, if someone grows up speaking English and German natively and then learns French at school, French is a second language to them, even though you could say it's language #3.
