Making large payment on Mortgage not lowering maturity date

tl;dr: if you see your loan balance has decreased by the amount of the additional payment, then your additional payments are working as intended. Don't worry if the maturity date is unchanged.

You must make sure that it is considered additional principal payment. Call them and make sure it gets adjusted if it isn’t.

Otherwise, the bank might consider it ‘pre-payment of your next so-many monthly rates’, and just put it aside for that without giving you any benefit. If you think that is a silly interpretation of a large payment, I agree, but it’s allowed and obviously advantageous to the bank, so they sometimes try to do it.

It would give you 15 years off if you prepay that much every year, but the relationship is not linear.
Your first extra payment will save you interest for that amount for the remaining lifetime; your second extra payment will save your interest for a shorter lifetime – because the lifetime of the mortgage is already shorter from the first extra payment. So the effect becomes ever smaller (but still good for you).

Update: just to mention, mortgage companies often don't show a different maturity date when you prepay principal. The date shown is supposed to be the 'original maturity date', and they don't bother to update it. You basically have to do your own math if you want to know when you are done. Rest assured though that the mortgage will end earlier if you prepay principal.

Anyway, it is a good idea to pre-pay, and you should continue doing it when you can!

I suspect that they're showing the maturity date as it is specified in the original mortgage, because 1) that's a legal document; and 2) it's easier to code their web site that way, rather than adjusting things every time you make an extra payment.

Take things to the limit, and assume you're paying off the mortgage in full (as when you sell or refinance). That doesn't change the maturity date of the loan, it just means you paid it off early.