Understanding この年にして in this sentence

Question

Grandad has just promised something he is unlikely to be able to fulfil. He then thinks to himself:

「いざとなったときもし逃げたりしたらわしゃこの年にして裏切り者のウソつき者じゃ。どうなるかわからんことまで言うもんじゃないのう。」
If push came to shove and I were to do something like run away, I would be a liar and a traitor. I shouldn't go as far as to say that I don't know what will happen.

I translated この年にして as 'at this age', but I don't see the relevance. If he fails to fulfil his promise he'll be a liar and traitor whatever his age. What have I misunderstood?

The second sentence also seems unconnected. I must be missing something here.

Answer 1

「この年{とし}にして」＝「この歳{とし}にして」＝ "at this age"

The nuance of the phrase in the context is that the speaker would not like to be a liar and traitor at his old age. That is to say he does not want to be like that at the last stage of his life.

Regarding your TL of the second sentence, you missed the structure around the middle part of it -- 「～～ことまで言う」.

There is no equivalent of "go as far as to say that" in the original. There is only "even talk about (the things you don't know)".

It looks as though you mistook the 「こと」 for the quotative 「と」. That would make a huge difference:

"I shouldn't be even talking about the things I don't know would (actually) happen." .

Answer 2

If he fails to fulfil his promise he'll be a liar and traitor whatever his age.

I think "at this age" is correct. Generally speaking, it is expected that older people have more wisdom and are less likely to do such things, which is why I think it is used.

The second sentence also seems unconnected. I must be missing something here.

If you break it down

どうなるかわからんこと (Something which you do not know the outcome)

まで (Used as emphasis)

言うもんじゃないのう (Saying you shouldn't say something)

So, the speaker is showing regret for what they said. Perhaps you could translate it as "I should never say things without fully understanding the outcome".