Youngs – who has more than 100 England caps and played for the Lions against Australia in 2013 – was part of a 50-man initial group contacted by coach Warren Gatland, to be whittled down ahead of the tour of South Africa.
“Whether or not I would have made the final cut, I’ll never know,” Youngs told the Daily Mail.
“I’ve got two young children and my wife is heavily pregnant with the third. This summer I’ve got an opportunity to be with my family and my kids.
“I have always loved the Lions. I’ll be cheering them on, and I wish everyone involved a healthy and successful tour.
“It’s not a decision that I’ve taken lightly, but it’s the best decision for my family.”
Gatland is set to name his 36-man squad and captain on Thursday, but a revised itinerary for South Africa made necessary by the coronavirus pandemic has yet to be published.
The Lions have a historic warm-up match against Japan scheduled at Murrayfield on June 26.
Additional reporting by PA.