I'm afraid they don't offer non-consecutive passes at concession prices, but the 65-69 price is a set price per day and doesn't get cheaper by buying a longer duration. So you can just purchase a day pass on the days you need. Please be advised (Information from Grandvalira):
4) Senior ski pass (65-69 years): Born between 1950 and 1954. Direct sale at ticket office. Official proof of age required. Price per day.
Hope this helps,
Ian
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