29 years old is not a bad age at all for a central defender. Thiago Silva, Chiellini, Godin, Sergio Ramos, Pique .. all still playing pretty much their best football in their 30's, some of them are even (gasp) 33. The best centre backs don't rely on pace but positioning and Alderweireld has never been quick, he's easily got a number of years left in him before he's ready for the glue factory and 50 million for one of the best defenders in the league isn't an unfair price. Players don't suddenly drop dead at 30, plenty of players in his position can play in to their mid 30's without serious decline. It's not like you don't have other young defenders at the club (Bailly, Lindelof) who can learn from him.
The market has changed, 50 mill isn't a shocking fee anymore. There are other question marks over Alderweireld (recent injuries) but his age is no problem. Vertonghen is 31 and playing better than ever, if you signed Alderweireld for 50 mill and he gave you even 3-4 seasons of top class defending he would easily justify the fee, and injuries permitting he's very capable of doing that. Nobody better than him is obtainable, you're not going to get Umtiti or Varane and whilst I rate Maguire he'd cost even more and whilst he's younger I'd argue he's just not as good a player.