The question you opened
Mia is three times as old
Mia is 3 times as old as her brother, who is 4 today.
How old will Mia be when her brother turns 10?
18 years old
Mia is 12 now, eight years ahead of her brother. That gap never changes, so at 10 he is eight years behind 18.
Mia is three times as old as her brother, who is four. When her brother turns ten, Mia will be eighteen. Start with today. Three times four is twelve, so Mia is twelve. Her brother is four. The difference between them is eight years. Now the important part. That difference of eight years is fixed forever. Mia was born eight years before her brother and nothing will change that. Every birthday he has, she has one too.
So when he is ten, she is ten plus eight, which is eighteen. The answer that feels natural is thirty, because three times ten is thirty. It assumes Mia stays three times older, and that is the thing ages never do. Being three times as old is a temporary fact about one particular day. When he was two she was six, three times as old. Today at four and twelve she is still three times as old. At ten and eighteen she is less than twice as old.
At forty and forty eight the gap will look tiny. You can see it clearly by writing the two ages side by side and adding a year to both, over and over. 4 and 12. 5 and 13. 6 and 14, and so on to 10 and 18. The gap is eight on every single line, and the multiple falls the whole way down. That is the rule to remember: when both numbers grow by the same amount, the difference stays and the ratio moves.
Once you have it, age puzzles become easy. Find the gap first. Everything else follows from it. If a puzzle tells you someone is twice as old now and asks about the future, work out the gap today, then carry it forward. Try one on someone at home. Ask how old they will be when you are twice as old as you are now, and see whether they reach for the gap or the multiple.
Technique: The age gap stays the same