Generally we encourage the men to write to women whose age is within 20 years of their age for Russia/Ukraine, Latin America, and China. For example: A 50 year old man has the most opportunity with a lady age 30 and above.
For the Philippines, the ladies are usually more flexible. Typically within 30 years of the man's age.
If you do not follow the guidelines outlined here, you take the risk of the lady not taking your intentions very seriously. They may be happy to write you, but may not have any ideas of getting seriously involved with you if you travel to their country. This is not true in all cases since every relationship is unique. However keep it in mind.