We're back to the main line of the story at this point. Isaac marries Rebekah when he is forty years old--we do not know her age anywhere along the path of the story. Her details are absent--but we know that she was 20 years older when the boys were born. We get that from seeing that Isaac was 40 at marriage and 60 at fatherhood--given that Rebekah was adult enough for her decision to count in marrying Isaac, she's certainly going to be mid-30s at childbirth. I'd guess she's mid-40s, but that's all it is. A guess.
And "guessing" isn't on the list of great ways to understand the truth of Scripture, so don't count that guess as worth much. We can be certain that it was still somewhat miraculous for her to have children, as 25:21 shows that Isaac and Rebekah definitely saw her pregnancy as a gift from God.
Rebekah has twin sons, Esau and Jacob. Esau is the firstborn but Jacob is right behind him. The developments of the next few chapters revolve around these two and their rivalry. The summary would appear to be that Esau doesn't really think through things...and Jacob schemes for the future. Hebrews 12:16 criticizes Esau for selling his birthright for a single meal.
We also see in this passage that Isaac picked up one of Father Abraham's not-so-good habits: while complimenting his wife's looks, he also persuades her to lie about being his sister. It's not a great habit--and it results in trouble with the locals.
The other thing that happens here is the migration of Isaac farther away from the Philistines. God tells him not to go all the way to Egypt, so instead he moves into the wilderness and digs new wells until he finds a place he can settle.
There's something to consider in his obedience about not going to Egypt--just a few years after his death, his descendants will relocate to Egypt. Yet if they had been there already, who knows if Joseph would have been in the position he held? Possibly not...
God works in our lives, sometimes in ways that we will never understand. Follow and trust.