Hm, tried this on a Mac but something must be askew - it returned a commit by me in 2022 in a repo that has existed since at least 2017.
git ls-files -z|xargs -0 -n 1 git blame --date=format:%Y%m%d -f |grep -Eo '\d{8}.*' |sort -n | head -n 1 | sed 's/^[^)]*)*[ \t]//'