^If you mean keeping them on the vine till they're to be eaten, tomatoes will ripen and sweeten over time even after picking, provided it's done after the colour starts to change and you don't refrigerate them. You shouldn't notice any difference between on- and off-the-vine ripened ones in a blind test. How sweet exactly they are may depend on the size they grow to, cultivar choice, growing conditions, and one's own taste buds.
If you mean compared to store-bought tomatoes, many of them taste bland due to production-transportation-storage methods (and at least in the USA and tomato-exporting countries, due to commercial cultivars meant for long-distance shipping sacrificing the taste for durability), not because of their age.