This is one of those "biggest in the world" claims that depend totally on definition, here on the definition of what constitutes "pound", "active" and "currency". The Frankish pound came before the English pound, so any currency called Franc today is older than the pound. However, if your claim is that the pound has precedence because its name comes from the Roman weight lira, any currency called dirham (drachme) today is older. If you define "currency" as a monetary system, the pound dates from 1968-1971 and the Indian rupee (1957) plus quite a few others are older. If "active" means struck (but not necessarily circulating) virtually unchanged today, the Dutch gold ducat would be a strong contender and easily beat out the pound and even the Swiss franc.
Peter