Is it considered rude to not ask to help a person with a walker?

Asking is nice (just a quick and casual "Can I help you?," but don't insist or be pushy; it might hurt their pride.

Besides, look at it pragmatically. What are you going to do, push the walker for them? I'll assume you were thinking of offering your arm for them to hold. When a person has so little agility, balance, or strength left that they've degenerated beyond a cane and must use a walker, it's very likely that they need far more support than your arm will provide. What are your other options? Carry them? Give them a piggyback ride? Even if they take your arm, what will they do with the walker? Drag it along behind them?

Probably the most helpful thing you could do is to hold the door for them or give them a boost over the curb with a gentle push from your hand under their arm.

It is considered rude to insist upon helping someone if they do not ask for help. However, it is not rude to not help someone if they do not ask for help.