Typically, it would appear that--as they grow older--your pink hydrangeas are showing their age by flowers fading to white. Fortunately, this is not a sign of age, but rather a change over time in the nutrients of your soil; you can restore your pink blooms by re-establishing soil balance. Experts counsel that color change in hydrangeas is directly related to their uptake of aluminum compounds in the soil. Changing the pH content of your soil (the measurement of the acid/alkaline composition) can turn fading white hydrangea blooms back to pink.
Consider soil testing, especially if your hydrangeas are a large, established planting. This will help you understand how much "sweetening" (alkanization) your soil needs. Purchase a small soil-test kit at a nursery or contact your USDA County Extension Service for testing materials and advice specific to your area.
Assess changes you may have made in plantings that affect your hydrangeas. Adding acid-loving evergreens and supporting them with acid-increasing fertilizers may have made aluminum compounds more accessible to hydrangea roots. If it is possible that you have made such changes in plantings and fertilizers, consider providing some barriers to separate hydrangeas from other plants. Using your trowel or space, dig a ring around each hydrangea 4 to 6 inches deep (use the leaf spread of the plant to determine the circumference of the ring). Place flexible lawn edging in the ring and cover lightly with soil. This will help to separate the nutrients that your hydrangeas need.
Trench to a depth of 4 inches around small or single plants; this may provide an adequate barrier without edging for single plants.
Apply lime within the protective ring around your hydrangeas several times a year. Once a season is a good guideline for keeping soil consistently sweet. Using a hand fork, loosen soil gently around the outside edges of the ring and mix lime with the top inch or two of soil to prevent it from being carried away by wind. Water the lime and soil mixture to help lime leach into the soil.
Consider increasing the blockage of aluminum salts by using a high-phosphorus fertilizer if regular lime applications do not produce desired results. (Hydrangeas are usually such vigorous growers that one hesitates to fertilize at all.) Fertilizers high in nitrogen will encourage massive leaf growth. Fertilizers high in phosphorus, however, directly address the issue of blossom color.