The home inspection contingency is to look for defects in a home. A defect is defined as something that can significantly threaten the safety of the home's occupants, the value of the home or the life of the home. Key word "significantly".
Only an attorney can tell you if what you stated was a defect, but I tips towards no. Both are working properly and apparently cause no threat as stated above. These would not be "walk away" items.
That said, you could ask for anything in your home inspection contingency, even if that were to pain the entire house pink. It is up to the seller to decide to do it for you. Were I the seller's agent, I would advise them that the equipment may be old but it is working as expected. They don't "need" to fix it to keep you in the deal. They likely won't fit the definition of defect either, so these items would probably not be firm grounds for canceling the deal.
My best advice to you is if the house came out great on the home inspection stay in the deal and budget for a new furnace and central air. If you maintain the existing units well, they may last a long, long time. The phrase your home inspector probably used is "at the end of their EXPECTED life". These things don't just croak after a certain amount of time. Maintain it and you could get many more years out of it.
Best of luck.