One of the most common causes of vaginal loosening is due to birth giving. The more a woman gives birth, the higher is the propensity for her vagina to slacken because her vaginal walls are stretched out.
Another cause of loosening of the vagina is due to frequent sexual activity. Men prefer tight vagina that is why they choose younger women as sexual partners while for women, having a tight vagina could intensify her feeling of arousal and would make her feel every thrust and pressure done by her partner.
There are different factors that could lead to the slackening of the vaginal wall and you cannot tell exactly if your vagina has loosened based on exact measurements but there are some indicators and they include the following:
The need to insert bigger objects into the vagina for stimulation and arousal
The difficulty to grip your index finger using your vagina
Your vagina no longer closes when you are not turned on
You can freely insert more than three fingers without much resistance
Difficulty to achieve orgasm
Failure to satisfy your partner unlike before