How old must the money be, and what does "source of funds" mean?

Short answer: The checklist asks for a 12 month statement, so the window is a year (Dhaka checklist). "Source of funds" means being able to document where significant amounts came from.

What a 12 month statement shows

Not the closing balance. The shape of the account over a year: income arriving, ordinary spending, and anything that does not fit the pattern.

What documenting a source looks like

If a large amount arrived, you should be able to point to the thing it came from, with paper: a salary, a business you can document, rent from a property you own, the sale of land with the deed, a gratuity or pension payment, a remittance from a relative whose own income accounts for it. Documents rather than explanations.

What the embassy flags

In its own framing: a large deposit shortly before the statement with no clear source, and a sponsor whose income cannot explain the balance.

What I do not know

Whether a particular deposit, of a particular size, on a particular date, will be queried. No threshold is published. Anyone who tells you "season it for three months and it is fine" is repeating a rumour, and it is a rumour about money, which is the part of the file that gets looked at hardest.

The honest version

If your family's money is real, document it properly and early. If it is not there yet, the answer is a later intake, not a better looking statement (V11).