My teeth sometimes get a dull ache at night. could this be due to lack of brushing teeth?

Possibly: At your age it is possible if you don't practice good and regular home care. Most often this is related to clenching and grinding the teeth. See if you catch yourself doing this and ask those around you to see if you are doing this. First, get on track with the brushing and flossing. Protect and keep what you have. Good Luck.

Dull ache at night: There are many possibilities, merely lack of brushing would not be at the top of my list as to cause. The best approach is to see a dentist for a complete dental exam and consultation. It is not in your best interests to answer these types of questions over the internet. When was you last exam?

Yes: It can also be due to grinding them together.

How gross: In unclean mouth looks and smells gross. Yes, tooth/gum infections can be painful. So can clenching or grinding, fractures, or other causes. Please see a Dentist to reestablish your oral health and for help in maintaining a smile that you can be proud of.