Avraham Avinu had his bris milah when he was ninety-nine years old. Masechta Geirim (4:3) says that this was so those interested in converting to Yiddishkeit shouldn't refrain from converting even later on in their lives. If Avraham would have had his milah when he was twenty, people above twenty would think that it is too late for them to convert. But he had his milah at ninety-nine, so they realize there is always time to convert.
The same applies to teshuvah. You can be later in life, but it isn't too late to do teshuvah. If Hakadosh Baruch Hu wanted that a non-Jew should come under the wings of the Shechinah even in his old years, certainly (kal v'chomer) Hakadosh Baruch Hu desires that His nation, Bnei Yisrael, should circumcise their heart in the later years of their life, because it is never too late to return to Hashem.
Rashi (Bereishis 32:23) teaches that Yaakov Avinu placed Dinah in a box because he feared Eisav might see and want to marry her. Rashi writes that Yaakov was punished for doing so because if Dinah had married Eisav, perhaps she would have brought him to do teshuvah. The punishment was that Dinah was captured by Shechem.
Reb Yechezkel Levinstein zt'l says that this Rashi shows how much Hashem waits for everyone to return to Him. At this time, Eisav was ninety-eight years old and he was a great rasha. Yet, Hashem was waiting for him to do teshuvah. He is undoubtedly waiting for us to do teshuvah.
The Zohar (Introduction 2: with commentary Masok Midvash) tells that when Hashem was about to create the world, all letters came to Hakadosh Baruch Hu, one by one, and each letter said that the world should be created with it. When the letter tzaddik came and claimed that the world should be created with it, Hakadosh Baruch Hu replied with love that indeed, the world deserves to be created with the letter tzaddik; however, if the world would be created with the letter tzaddik, which represents immense kedushah, people will fall into yeush. They will feel that they can't correct what they did wrong if they fall into sin. Hakadosh Baruch Hu didn't want to cause His creations to lose hope. Therefore, Hashem didn't want to create the world with the letter tzaddik, although it was befitting. The main thing is that people shouldn't lose hope.
The Zohar states that the world was created with the letter 'ב (and this is the reason 'ב is the first letter of the Torah). The sefarim say that the letter beis is closed from all sides and open on its front side. This hints that a person shouldn't think about what happened in the past; he should focus on the future to make the future better. So, the letter 'ב represents that we shouldn't lose hope and can improve the future. No matter what was, when we make the future better, everything becomes good, and even the past becomes rectified in the best way.
A rather recent invention is a Shabbos lamp. It is a patent that enables a person to have light on Shabbos when he wants it, and darkness when he wants it. The lightbulb is in a box. When you close the box, you don't see the light, and you open the box when you want the light. Obviously, even when the box is shut, the light is still on; you just don't see it. This is a mashal to a Yid's neshamah. Sometimes you don't see that the neshamah is shining, but the spark is always there. You don't have to create something to turn the neshamah on because it is already shining brightly. It shines brightly and desires only Torah, mitzvos, and a connection with Hashem. All you have to do is to move the disturbance away (the pull to the temptations of this world which prevents us from recognizing the light of the neshamah) and then the light will shine through.
