In order to provide a personal response to a question posed about a poem, one needs to define what the poem says to them and how they feel about the poem.
Given that this is a question regarding a "personal response" to Digging by Seamus Heaney, the answer received will provide my own (subjective) idea. That being said, I hope that my answer will help you to form a "personal response" of your own.
Therefore, the concept of age, while not blatant, can be seen in the poem. The young man (the narrator and assumedly Heaney himself) chooses to take up a pen instead of the spade his father and grandfather have both taken up. What this says about youth and old age is that what is good for some is not good for all.
The fact that Heaney deacribes his pen as a gun is curious. ONe could easily believe that his father does not look at the pen as work, like he does the spade. By naming the pen as a gun, the narrator is stating that he is ready for a fight and/or ending the manual labor of his family line by taking up the pen.
Overall, it seems to me that the narrator respects the work of his father and grandfather ("By God, the old man could handle a spade"). Regardless of the respect, the narrator has no "spade." This can seen as both symbolic and literal.
Therefore, in the end, my idea of the picture of youth and old age is that the new (the youth) have respect for those who have come before them, but they fail to possess the same ideologies as those who came before them. Instead, they choose their own spade as a representation of the respect and their new way of doing things.