You can simply switch the value: start_year with the value: end_year to have
<%= f.input :date_of_birth, :as => :date, :start_year => Date.today.year - 12, :end_year => 1900, :order => [ :day, :month, :year], :required => true %>
Like: start_year and: end_year - this is exactly how you show the year ...
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