In the below scripture, God promises Abraham (then called Abram) a child of his own.
Genesis 15:2-6 (NKJV) 2 But Abram said, “Lord God, what will You give me, seeing I go childless, and the heir of my house is Eliezer of Damascus?” 3 Then Abram said, “Look, You have given me no offspring; indeed one born in my house is my heir!” 4 And behold, the word of the Lord came to him, saying, “This one shall not be your heir, but one who will come from your own body shall be your heir.” 5 Then He brought him outside and said, “Look now toward heaven, and count the stars if you are able to number them.” And He said to him, “So shall your descendants be.” 6 And he believed in the Lord, and He accounted it to him for righteousness.
Abram did believe God, however the promise was years in coming (way past beyond child bearing years), so Abram & Sarai tried to fulfill God’s promise under their own power.
Sarai told Abram to go into her servant that she might bear him a child. Abram did just that, but since he went under his own power, the child that Sarai’s maid servant bore him was not the son that God promised him would be a great nation. In fact, Ishmael had to be sent away and would become a thorn in the side of the nation of Israel. To this day there is war between the Arab nation (Ishmael’s descendants) and Israel (Isaac’s descendants).
Genesis 16:11-12 (NKJV) 11 And the Angel of the Lord said to her: “Behold, you are with child, And you shall bear a son. You shall call his name Ishmael, Because the Lord has heard your affliction. 12 He shall be a wild man; His hand shall be against every man, And every man’s hand against him. And he shall dwell in the presence of all his brethren.”
When we deal outside of God’s authority, we may get what we want, but in the long run, it could be a problem.
Wait on the Lord for as long as it takes, because even though it might seem like a lifetime to you, God does everything in the right time and your reward will be great.