Yes definitely it is okay especially if you find it more easy that way. But the Sunnah practice is on the second method which -Allah knows best - might be more rewardful. Also with the second method of day-by-day, suppose you die before completing all those years. You can present to Allah all those completed days and your pure intention for missed days and ask for His forgiveness. But again, both methods are okay. You should start counting it from your puberty day onwards. You are obliged or held responsible for salah before the puberty. Exact ages although would vary from person to person, girls experience it at 9, and boys at 12 as lower limit. Upper limit for girls is 17, for boys 18. According to Imam Abu Yusuf and Imam Muhammad, the upper limit for both genders is the age of 15. If you don't remember your exact puberty date, you can go with the safer lower limit of 12. That means you should make up 9 years. Hope that helps.