(please check) fo meritnation experts of english

A sentance you have given is wrong, you gave

"we are the oldest members of the club"

its actually

"we are the eldest members of this club

The sentence provided at our website is correct. The word 'oldest' here is used as the superlative of 'old' which conveys the meaning that 'We are the oldest members of the club.' Hope your doubt is cleared.