Make a list, or a spreadsheet if you can.
Work out the monthly household costs, include everything, council tax, insurances, water charges, broadband, TV packages, utilities,mortgage if you have one, include all household expenses such as communal food, cleaning materials, window cleaning, toilet paper, car costs if you have one and they have use of it, be very thorough.
Multiply the total by 12 and divide by 52, then divide again by the number of adults. This will give you the approximate weekly household expense to sustain each adult. Use this figure as a starting point for discussion.
Explain to them all, though maybe not the 16 year old, that these are the expenses that need to be paid out to support everyone's comfort and wellbeing. Explain that you are not looking to make a profit from them, or to live for free yourself, but that as adults enjoying the comfort of living in the house they have to make a reasonable , cash not kind ,contribution towards the household. Or move.