The mean specifically different things. She was the love of my youth - this refers to someone that you loved when you were young. You might not love, or even know them anymore She was my youthful love - this means that she is young. Perhaps a better statement would be “We fell in love together when we were young, and have loved each other ever since.”

I would say, She was my childhood love(or sweet heart)

By themselves neither are very natural or precise. ‘Youth’ is a vague category which can go as low as age 10 in some contexts, maybe 15 in others. Similarly ‘love’ as a noun could refer to attraction/fondness but also being lovers.

If you mean to say that she was your love when you were young, “the love of my youth” is the better and more natural choice. In “She was my youthful love,” the adjective “youthful” describes “love” rather than the time period. This could mean the love itself was full of youthful qualities (e.g., innocence, passion, or inexperience), rather than necessarily referring to when it happened or how old she was.