An important thing to know when introducing the food..
If you introduce new food to your pleco and they are not eaten overnight.. it doesn't not mean that they do not eat that type of food. It only means that they are unfamiliar with what they are accustom to and will take time to learn that it is food. It may take up to 3-4 days for the pleco to learn and understand that it is a food source. In addition, it is easier to have a few well established plecos in your tank and introduce other tankmates. This will increase the plecos chance of finding and eating, they learn from each other.
My thoughts on different food is that: cucumbers are great food source, but are not the only kind and should not be feed 100% of the time.
Watermelon, honeydew and cantaloupe are some of great source of food for Plecos, but they can easily cloud your water overnight. Potato is easier for plecos to feed, but should only be inside the tank for no more than 3 days. They are also a better alternative for Panaque type fish in addition to cucumbers and wood.
For carnivorous plecos, shrimps and fish fillet are good examples.
With that said, hikari algae walfers are good examples of food, but Cichlid Gold sinking type and carnivorous sinking pellet are also great food.
To answer the previous question.. Pumpkin would be good for young and old.. . although when plecos are young, they do need more nutrients. Personally I have not tried pumpkins since they are seasonal vegetables, but I they are similar to cantaloupe and i would think they might cloudy the water.
I have also heard a Mod with a Royal pleco that died due to overfeeding on a pumpkin