What you're looking for is object accessors. Check it out in the official docs.
//Your code will probably look something like this in the end:
...
function selectYear(year){
return taxSlab[year];
}
function selectAgeOrBelow(obj, age){
//If an exact value exists. If not, you have to go through the whole
//object using Object.entries or hasOwnProperty and return the smallest
//value. That can then be returned instead.
return obj[age]; //an array
}
//And you can use them like:
let userObject = selectAgeOrBelow(selectYear("2019_20"), "60"); //an array
//Of course, you can use all of that in one line if you prefer (no need for multiple functions, but it helps)