Prayer (salah) becomes obligatory from the age of puberty. Let's say your puberty came at the age of 13 years, and now you are 30 years of age. So you must have 17 years of prayer from puberty.
So if you left prayer for sometime, you need to complete the unfulfilled prayers (with qada salah) totalling 17 years.
Now there is a difference of opinion -
Case A) Hambali madhab:
Imam Ahmed ibn Hambal (Rahimahullah) said one who left salah, left Islam. He (ra) has based this on this hadith,
Prophet (peace be upon him) said, "The difference between us and them is salaah.
Whoever neglects it is a kaafir." (Reported by Imaam Ahmad, Sahih Hadith)
This is also the position of Hambali madhab. There are few other hadiths to support this.
So that means, if you left salah completely and intentionally for a few years and started to do again at the age of 25, then you basically became Muslim again. So you do not have to do qada salah for all the neglected years.
Case B) Majority Scholars on Scenario-1
If a person denies that prayer is obligatory and so he did not do the salah for years, then he is a kafir, and his case is same as case A above.
Case C) Majority Scholars on Scenario-2
If a person does not deny that prayer is obligatory, but he missed prayers due to laziness, then he is still a Muslim with grave sin, and he should complete the missed salah.
Variation of Case C based on burden:
If the person has missed many many years of prayer, for example, 15 years of prayer. This can be a huge burden for him, and it maybe said to him - 'repent to Allah with sincerity, and complete from the missed prayer as much as you can, and pray all the sunnah prayers connected to the five obligatory prayers and hope for Allah's mercy to shower upon you for the incomplete prayers on the day of judgement.'
But if the person thinks he can fulfill all the years of missed salah, even if it is difficult, then he has taken a safe path - May Allah make it easy for him. Ameen.
Allah knows best.