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How old are your late payments? Your scores will improve as the late payments age & drop off of your reports, & as your positive history grows.
How many lates are we talking about? Are they just 30-day lates, or more serious? 30-day lates shouldn't be hurting your score quite as much after 2 years, but if you have 60 or 90-day lates, those are probably keeping your scores down. Combined with having very little & very new positive credit, it could be a little while before you see a vast improvement in your scores.
Usually after 2 years have passed since your most recent late payment, your scores are affected very minimally. My last and only 30 day late was about 2 and a half years ago and I noticed improvement in my scores after the 2 year mark. Then again, this all depends on how many lates, whether they are 30-60-90, and how much new good credit you have reporting. To keep it simple, have good revolving credit with utilization below 15% and continue making your payments on time. After 2 years you should at least be in the 600's with your score.