This is sad on both fronts. Consider that your 75 year old parents never thought they would be living these conditions themselves. How hard it must be for all of you.

What kind of arrangements do you have with them? In what manner do you contribute?

The answer to all of this may be to pay rent (which in my opinion you ought to be doing). This gives you a great deal more rights. It might even earn you more respect from your parents. However, until that happens it is a case of 'my house my rules' and that is something you can't escape from. This you need to respect.

Once/if the matter of rent has been settled draw up with them a list of boundaries and expectations for both sides to follow. As a tenant they have obligations towards you.

I sympathize with most of what you have to say. Your example of cutting the strawberries on a plate though made me cringe. I admit I am with your parents on that one as it doesn't demonstrate common sense. Perhaps that is the litmus test to your situation.

Finally, to what advantage do you have living under their roof? I am sure there are programs like low income housing subsidies available (my own income is topped up by one). Visit your local mental health association and social services centre. They likely have lists of available low income rentals that they update daily. They also will be able to advise you on what resources and programs may be available to you (I get subsidized transit passes, free leisure passes, and community activities on a sliding scale).

Having you live under their roof is obviously coming at a financial cost to them. They may very well agree to use the same funds to subsidize living outside of their home.