Greetings! Sun exposure can cause skin tanning and premature ageing. Skin tanning occur after short period of exposure but for premature ageing it takes many years. In my view your assumption regarding loss of 50% collagen is wrong. As for as for as regaining formal glory, you have to avoid sun exposure as much as possible (esp during 10AM to 4PM). Use sun protection like wearing hat and applying sun screen properly if you have go outside. In addition doing regular exercise, eating health diet and drinking 2-3 litres of water every day has positive effect on the skin. Our skin has a remarkable capability to repair any damage especially in early stages.