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Living past 90 is by no means rare.
Literal
90 years or-more live thing [topic-は] never rare [copula not-では].
決して~ない ('never, by no means') again pairs with negation, here ではない ('is not'). 以上 means 'X or more.' まれ ('rare') is a na-adjective, negated as まれではない. The whole construction double-hedges: 'is not rare' + 'by no means' = 'is absolutely not rare' = 'is quite common.' A typical Japanese way to express affirmation via double negation and understatement.