Assuming a month is 30 days, then you could use this formula in D1:

="Less than "&INDEX({3,6,12,24,36,48},MATCH(INT((TODAY()-A1)/30)+1,{0,3,6,12,24,36}))&" months"

then copy down. If your new data is only over-writing column A, then the formulae will remain intact.

Hope this helps.

Pete