It has to include pensioners. To make it work you need to take the cost of the bureaucracy out of the system, which means UBI supersedes all other welfare. There will be no pensions, no unemployment, no child benefits (children also get UBI, paid to their parents until they come of age), no means testing - everyone gets UBI, full stop. That's what the U is for!
Interestingly until recently Children in the UK got UBI in the form of family allowance (paid to the parents). x for first born, y for subsequent or something. The sweeping government cuts lead to it becoming means-tested and complicated.
One Conservative (sorry cannot find the quote) remarked that they spent theirs on Champangne, which caused horror in the press, but is money flowing into the economy at least...unlike tax benefits to land-owners
But since you want to keep social security, medicaid and medicare you need to keep a budget that somehow has 2 trillion dollars for BUI and 2 trillion for pensioners and children.
Not much of an improvement really.
Which is my main issue with everyone who proposes UBI. Once they are shown the number you get by multiplying population by income they slowly start to reinvent the welfare state that somehow is so terrible it needs to be replaced with UBI.
As someone once said: Crypto-currencies are gods way of teaching libertarians why regulations exist in the financial sector. I would like to rephrase that: UBI is gods way of teaching libertarians why the welfare state was created.
Also I don’t know which version of libertarian you’re using, but I don’t think I’d classify libertarians as being super supportive of UBI...
I've seen arguments UBI should include pensioners, but never for it to include minors.