Assalamualaikum ustaz. I never prayed since I reached puberty until I was 22 years old. From my understanding, I have to replace all of my missed prayers. I want to ask, is it permissible for me to perform sunnah prayers during the time I’m replacing my prayers? Or just some sunnah prayers are allowed? Hope for an explanation.
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
Allah SWT states:
قَدْ أَفْلَحَ الْمُؤْمِنُونَ ﴿١﴾ الَّذِينَ هُمْ فِي صَلَاتِهِمْ خَاشِعُونَ
“Certainly, will the believers have succeeded: They who are during their prayer humbly submissive.”
(Surah al-Mukminun: 1-2)
Ibn Kathir when commenting on the above verse said: Ali bin Abi Talhah narrated from Ibn Abbas R.anhuma: “خَاشِعُونَ” (they are the humbly submissive) who are those who fear Allah and with complete calmness. From Ali bin Abi Talib RA: What is meant by humbly submissive here is the condition of one’s heart. (See Tafsir al-Quran al-‘Azim, 5/110)
First and foremost, we’d like to congratulate you on your concern and care on the importance of prayer when most are neglecting theirs today. Islam truly emphasizes prayer. From Abdullah bin Buraidah, from his father R.Anhuma, Rasullullah PBUH said:
العَهْدُ الَّذِي بَيْنَنَا وَبَيْنَهُمُ الصَّلَاةُ، فَمَنْ تَرَكَهَا فَقَدْ كَفَرَ
“The covenant between us and them is prayer, so if anyone abandons it he has become an infidel.” 
From Mu’az bin Jabal RA, the Prophet PBUH said:
رَأْسُ الأَمْرِ الإِسْلاَمُ ، وَعَمُودُهُ الصَّلاَةُ
“The root of this matter (foundation) is Islam, its pillar (mainstay is) As-Salat (the prayer),” 
It is claimed, Rasullullah PBUH said:
مَنْ تَرَكَ الصَّلَاةَ فَلَا دِينَ لَهُ، وَالصَّلَاةُ عِمَادُ الدِّينِ
“Whoever abandons his prayer, then there is no religion for him and the prayer is the pillar of religion.” 
Prayer is honoured worship in Islam. Thus, Islam emphasizes obligatory prayer regardless of where and what the situation is. This obligation must be fulfilled no matter the situation when the obligatory conditions are met.
Syarak also wants us to teach our children to pray when they are still children. Rasullullah PBUH said:
مُرُوا أَوْلَادَكُمْ بِالصَّلَاةِ وَهُمْ أَبْنَاءُ سَبْعِ سِنِينَ، وَاضْرِبُوهُمْ عَلَيْهَا، وَهُمْ أَبْنَاءُ عَشْرٍ
“Command your children to perform Salat (prayer) when they are seven years old, and beat them for (not offering) it when they are ten and do not let (boys and girls) sleep together.” 
If we had made a mistake in our past and missed our obligatory prayers, then we should try our best to replace our missed prayers for it is considered as our debt that must be settled. Rasullullah PBUH said:
فَدَيْنُ اللَّهِ أَحَقُّ أَنْ يُقْضَى
“So, the debt due to Allah is the one which most deserves to be paid.” 
In the above issue, in our opinion it is best to hasten in replacing the obligatory prayer for you in terms of worship, however, it is permissible for you to perform sunnah prayer specified for a reason such as Dhuha and Tahiyyatul Masjid prayers. It is also advised for you to increase your charity, istighfar, dhikr and good deeds to others. May Allah forgive and bless us all.
Let us all supplicate with the following prayer:
رَبِّ اجْعَلْنِي مُقِيمَ الصَّلَاةِ وَمِن ذُرِّيَّتِي ۚ رَبَّنَا وَتَقَبَّلْ دُعَاءِ ﴿٤٠﴾ رَبَّنَا اغْفِرْ لِي وَلِوَالِدَيَّ وَلِلْمُؤْمِنِينَ يَوْمَ يَقُومُ الْحِسَابُ
“My Lord, make me an establisher of prayer, and [many] from my descendants. Our Lord, and accept my supplication.”
 Narrated by al-Baihaqi (2550) in Syu’ab al-Iman. Imam al-Suyuthi evaluated this hadith as dhaif. (See al-Jami’ al-Saghir, 2/64). Imam al-Sakhawi stated this hadith is narrated by al-Baihaqi with a dhaif sanad. (See al-Mughni, 1/174) Imam al-Sakhawi evaluated the sanad of this hadith as dhaif. He also cited: Imam Ibn Solah who said this hadith is unknown in Musykil al-Wasith and Imam al-Nawawi ruled this hadith as munkar and bathil. (See al-Maqasid al-Hasanah, pg. 427)