I don't think we get to a point when we're "too old" to say yes when we mean no. I think we were born too old for it.
I, for one, was shocked when I realized that, for all my rebellious, anti-authority tendencies, I was a person who said 'yes' against my better judgment, rather then hurt someone. That's not a habit leading to good, honest, and long-standing relationships. And while that hard 'no' is still difficult to articulate--for all of us--compromising yourself in your soul is far harder and will wear you out far more quickly than you realize.
Post-Independence Day seems as good a time as any to declare ourselves free from people-pleasing, free from worrying about others' feelings more than our own (which doesn't mean we don't have to be kind and considerate, just that, if we have to live with our decisions, they'd better be ones that put us right in our soul), free to say no when we mean no, and free never, ever to feel guilty about it before, during, or afterward.