I don't think you can say with any certainty that he's got 5 years left at his best. Not all players age well, particular those that have suffered a lot of injuries through their career like Kane has.
He could play on for 4-5 years still scoring 20+ goals per season or he could have 18 months of peak football in him before his body starts deteriorating on him. Given that he'll demand a 5 year deal earning £10m+ per year + signing on fees & agent fees, you're looking at a near £200m commitment to sign him. Again, you could get 5 years of good football out of him but it's a big gamble.
He could play on for 4-5 years still scoring 20+ goals per season or he could have 18 months of peak football in him before his body starts deteriorating on him. Given that he'll demand a 5 year deal earning £10m+ per year + signing on fees & agent fees, you're looking at a near £200m commitment to sign him. Again, you could get 5 years of good football out of him but it's a big gamble.